Documentation/git-read-tree.adoc - git - Git at Google


            
            git-read-tree(1)

            
            ================


            NAME

            
            ----

            
            git-read-tree - Reads tree information into the index


            SYNOPSIS

            
            --------

            
            [verse]

            
            'git read-tree' [(-m [--trivial] [--aggressive] | --reset | --prefix=<prefix>)

            
            		[-u | -i]] [--index-output=<file>] [--no-sparse-checkout]

            
            		(--empty | <tree-ish1> [<tree-ish2> [<tree-ish3>]])


            DESCRIPTION

            
            -----------

            
            Reads the tree information given by <tree-ish> into the index,

            
            but does not actually *update* any of the files it "caches". (see:

            
            linkgit:git-checkout-index[1])


            Optionally, it can merge a tree into the index, perform a

            
            fast-forward (i.e. 2-way) merge, or a 3-way merge, with the `-m`

            
            flag.  When used with `-m`, the `-u` flag causes it to also update

            
            the files in the work tree with the result of the merge.


            Only trivial merges are done by 'git read-tree' itself.  Only conflicting paths

            
            will be in an unmerged state when 'git read-tree' returns.


            OPTIONS

            
            -------

            
            -m::

            
            	Perform a merge, not just a read.  The command will

            
            	refuse to run if your index file has unmerged entries,

            
            	indicating that you have not finished a previous merge you

            
            	started.


            --reset::

            
            	Same as -m, except that unmerged entries are discarded instead

            
            	of failing.  When used with `-u`, updates leading to loss of

            
            	working tree changes or untracked files or directories will not

            
            	abort the operation.


            -u::

            
            	After a successful merge, update the files in the work

            
            	tree with the result of the merge.


            -i::

            
            	Usually a merge requires the index file as well as the

            
            	files in the working tree to be up to date with the

            
            	current head commit, in order not to lose local

            
            	changes.  This flag disables the check with the working

            
            	tree and is meant to be used when creating a merge of

            
            	trees that are not directly related to the current

            
            	working tree status into a temporary index file.


            -n::

            
            --dry-run::

            
            	Check if the command would error out, without updating the index

            
            	or the files in the working tree for real.


            -v::

            
            	Show the progress of checking files out.


            --trivial::

            
            	Restrict three-way merge by 'git read-tree' to happen

            
            	only if there is no file-level merging required, instead

            
            	of resolving merge for trivial cases and leaving

            
            	conflicting files unresolved in the index.


            --aggressive::

            
            	Usually a three-way merge by 'git read-tree' resolves

            
            	the merge for really trivial cases and leaves other

            
            	cases unresolved in the index, so that porcelains can

            
            	implement different merge policies.  This flag makes the

            
            	command resolve a few more cases internally:

            
            +

            
            * when one side removes a path and the other side leaves the path

            
              unmodified.  The resolution is to remove that path.

            
            * when both sides remove a path.  The resolution is to remove that path.

            
            * when both sides add a path identically.  The resolution

            
              is to add that path.


            --prefix=<prefix>::

            
            	Keep the current index contents, and read the contents

            
            	of the named tree-ish under the directory at `<prefix>`.

            
            	The command will refuse to overwrite entries that already

            
            	existed in the original index file.


            --index-output=<file>::

            
            	Instead of writing the results out to `$GIT_INDEX_FILE`,

            
            	write the resulting index in the named file.  While the

            
            	command is operating, the original index file is locked

            
            	with the same mechanism as usual.  The file must allow

            
            	to be rename(2)ed into from a temporary file that is

            
            	created next to the usual index file; typically this

            
            	means it needs to be on the same filesystem as the index

            
            	file itself, and you need write permission to the

            
            	directories the index file and index output file are

            
            	located in.


            --recurse-submodules::

            
            --no-recurse-submodules::

            
            	Using --recurse-submodules will update the content of all active

            
            	submodules according to the commit recorded in the superproject by

            
            	calling read-tree recursively, also setting the submodules' HEAD to be

            
            	detached at that commit.


            --no-sparse-checkout::

            
            	Disable sparse checkout support even if `core.sparseCheckout`

            
            	is true.


            --empty::

            
            	Instead of reading tree object(s) into the index, just empty

            
            	it.


            -q::

            
            --quiet::

            
            	Quiet, suppress feedback messages.


            <tree-ish#>::

            
            	The id of the tree object(s) to be read/merged.


            MERGING

            
            -------

            
            If `-m` is specified, 'git read-tree' can perform 3 kinds of

            
            merge, a single tree merge if only 1 tree is given, a

            
            fast-forward merge with 2 trees, or a 3-way merge if 3 or more trees are

            
            provided.


            Single Tree Merge

            
            ~~~~~~~~~~~~~~~~~

            
            If only 1 tree is specified, 'git read-tree' operates as if the user did not

            
            specify `-m`, except that if the original index has an entry for a

            
            given pathname, and the contents of the path match with the tree

            
            being read, the stat info from the index is used. (In other words, the

            
            index's stat()s take precedence over the merged tree's).


            That means that if you do a `git read-tree -m <newtree>` followed by a

            
            `git checkout-index -f -u -a`, the 'git checkout-index' only checks out

            
            the stuff that really changed.


            This is used to avoid unnecessary false hits when 'git diff-files' is

            
            run after 'git read-tree'.


            Two Tree Merge

            
            ~~~~~~~~~~~~~~


            Typically, this is invoked as `git read-tree -m $H $M`, where $H

            
            is the head commit of the current repository, and $M is the head

            
            of a foreign tree, which is simply ahead of $H (i.e. we are in a

            
            fast-forward situation).


            When two trees are specified, the user is telling 'git read-tree'

            
            the following:


                 1. The current index and work tree is derived from $H, but

            
            	the user may have local changes in them since $H.


                 2. The user wants to fast-forward to $M.


            In this case, the `git read-tree -m $H $M` command makes sure

            
            that no local change is lost as the result of this "merge".

            
            Here are the "carry forward" rules, where "I" denotes the index,

            
            "clean" means that index and work tree coincide, and "exists"/"nothing"

            
            refer to the presence of a path in the specified commit:


            ....

            
            	I                   H        M        Result

            
                   -------------------------------------------------------

            
                 0  nothing             nothing  nothing  (does not happen)

            
                 1  nothing             nothing  exists   use M

            
                 2  nothing             exists   nothing  remove path from index

            
                 3  nothing             exists   exists,  use M if "initial checkout",

            
            				     H == M   keep index otherwise

            
            				     exists,  fail

            
            				     H != M


                    clean I==H  I==M

            
                   ------------------

            
                 4  yes   N/A   N/A     nothing  nothing  keep index

            
                 5  no    N/A   N/A     nothing  nothing  keep index


                 6  yes   N/A   yes     nothing  exists   keep index

            
                 7  no    N/A   yes     nothing  exists   keep index

            
                 8  yes   N/A   no      nothing  exists   fail

            
                 9  no    N/A   no      nothing  exists   fail


                 10 yes   yes   N/A     exists   nothing  remove path from index

            
                 11 no    yes   N/A     exists   nothing  fail

            
                 12 yes   no    N/A     exists   nothing  fail

            
                 13 no    no    N/A     exists   nothing  fail


            	clean (H==M)

            
                   ------

            
                 14 yes                 exists   exists   keep index

            
                 15 no                  exists   exists   keep index


                    clean I==H  I==M (H!=M)

            
                   ------------------

            
                 16 yes   no    no      exists   exists   fail

            
                 17 no    no    no      exists   exists   fail

            
                 18 yes   no    yes     exists   exists   keep index

            
                 19 no    no    yes     exists   exists   keep index

            
                 20 yes   yes   no      exists   exists   use M

            
                 21 no    yes   no      exists   exists   fail

            
            ....


            In all "keep index" cases, the index entry stays as in the

            
            original index file.  If the entry is not up to date,

            
            'git read-tree' keeps the copy in the work tree intact when

            
            operating under the -u flag.


            When this form of 'git read-tree' returns successfully, you can

            
            see which of the "local changes" that you made were carried forward by running

            
            `git diff-index --cached $M`.  Note that this does not

            
            necessarily match what `git diff-index --cached $H` would have

            
            produced before such a two tree merge.  This is because of cases

            
            18 and 19 -- if you already had the changes in $M (e.g. maybe

            
            you picked it up via e-mail in a patch form), `git diff-index

            
            --cached $H` would have told you about the change before this

            
            merge, but it would not show in `git diff-index --cached $M`

            
            output after the two-tree merge.


            Case 3 is slightly tricky and needs explanation.  The result from this

            
            rule logically should be to remove the path if the user staged the removal

            
            of the path and then switching to a new branch.  That however will prevent

            
            the initial checkout from happening, so the rule is modified to use M (new

            
            tree) only when the content of the index is empty.  Otherwise the removal

            
            of the path is kept as long as $H and $M are the same.


            3-Way Merge

            
            ~~~~~~~~~~~

            
            Each "index" entry has two bits worth of "stage" state. stage 0 is the

            
            normal one, and is the only one you'd see in any kind of normal use.


            However, when you do 'git read-tree' with three trees, the "stage"

            
            starts out at 1.


            This means that you can do


            ----------------

            
            $ git read-tree -m <tree1> <tree2> <tree3>

            
            ----------------


            and you will end up with an index with all of the <tree1> entries in

            
            "stage1", all of the <tree2> entries in "stage2" and all of the

            
            <tree3> entries in "stage3".  When performing a merge of another

            
            branch into the current branch, we use the common ancestor tree

            
            as <tree1>, the current branch head as <tree2>, and the other

            
            branch head as <tree3>.


            Furthermore, 'git read-tree' has special-case logic that says: if you see

            
            a file that matches in all respects in the following states, it

            
            "collapses" back to "stage0":


               - stage 2 and 3 are the same; take one or the other (it makes no

            
                 difference - the same work has been done on our branch in

            
                 stage 2 and their branch in stage 3)


               - stage 1 and stage 2 are the same and stage 3 is different; take

            
                 stage 3 (our branch in stage 2 did not do anything since the

            
                 ancestor in stage 1 while their branch in stage 3 worked on

            
                 it)


               - stage 1 and stage 3 are the same and stage 2 is different take

            
                 stage 2 (we did something while they did nothing)


            The 'git write-tree' command refuses to write a nonsensical tree, and it

            
            will complain about unmerged entries if it sees a single entry that is not

            
            stage 0.


            OK, this all sounds like a collection of totally nonsensical rules,

            
            but it's actually exactly what you want in order to do a fast

            
            merge. The different stages represent the "result tree" (stage 0, aka

            
            "merged"), the original tree (stage 1, aka "orig"), and the two trees

            
            you are trying to merge (stage 2 and 3 respectively).


            The order of stages 1, 2 and 3 (hence the order of three

            
            <tree-ish> command-line arguments) are significant when you

            
            start a 3-way merge with an index file that is already

            
            populated.  Here is an outline of how the algorithm works:


            - if a file exists in identical format in all three trees, it will

            
              automatically collapse to "merged" state by 'git read-tree'.


            - a file that has _any_ difference what-so-ever in the three trees

            
              will stay as separate entries in the index. It's up to "porcelain

            
              policy" to determine how to remove the non-0 stages, and insert a

            
              merged version.


            - the index file saves and restores with all this information, so you

            
              can merge things incrementally, but as long as it has entries in

            
              stages 1/2/3 (i.e., "unmerged entries") you can't write the result. So

            
              now the merge algorithm ends up being really simple:


              * you walk the index in order, and ignore all entries of stage 0,

            
                since they've already been done.


              * if you find a "stage1", but no matching "stage2" or "stage3", you

            
                know it's been removed from both trees (it only existed in the

            
                original tree), and you remove that entry.


              * if you find a matching "stage2" and "stage3" tree, you remove one

            
                of them, and turn the other into a "stage0" entry. Remove any

            
                matching "stage1" entry if it exists too.  .. all the normal

            
                trivial rules ..


            You would normally use 'git merge-index' with supplied

            
            'git merge-one-file' to do this last step.  The script updates

            
            the files in the working tree as it merges each path and at the

            
            end of a successful merge.


            When you start a 3-way merge with an index file that is already

            
            populated, it is assumed that it represents the state of the

            
            files in your work tree, and you can even have files with

            
            changes unrecorded in the index file.  It is further assumed

            
            that this state is "derived" from the stage 2 tree.  The 3-way

            
            merge refuses to run if it finds an entry in the original index

            
            file that does not match stage 2.


            This is done to prevent you from losing your work-in-progress

            
            changes, and mixing your random changes in an unrelated merge

            
            commit.  To illustrate, suppose you start from what has been

            
            committed last to your repository:


            ----------------

            
            $ JC=`git rev-parse --verify "HEAD^0"`

            
            $ git checkout-index -f -u -a $JC

            
            ----------------


            You do random edits, without running 'git update-index'.  And then

            
            you notice that the tip of your "upstream" tree has advanced

            
            since you pulled from him:


            ----------------

            
            $ git fetch git://.... linus

            
            $ LT=`git rev-parse FETCH_HEAD`

            
            ----------------


            Your work tree is still based on your HEAD ($JC), but you have

            
            some edits since.  Three-way merge makes sure that you have not

            
            added or modified index entries since $JC, and if you haven't,

            
            then does the right thing.  So with the following sequence:


            ----------------

            
            $ git read-tree -m -u `git merge-base $JC $LT` $JC $LT

            
            $ git merge-index git-merge-one-file -a

            
            $ echo "Merge with Linus" | \

            
              git commit-tree `git write-tree` -p $JC -p $LT

            
            ----------------


            what you would commit is a pure merge between $JC and $LT without

            
            your work-in-progress changes, and your work tree would be

            
            updated to the result of the merge.


            However, if you have local changes in the working tree that

            
            would be overwritten by this merge, 'git read-tree' will refuse

            
            to run to prevent your changes from being lost.


            In other words, there is no need to worry about what exists only

            
            in the working tree.  When you have local changes in a part of

            
            the project that is not involved in the merge, your changes do

            
            not interfere with the merge, and are kept intact.  When they

            
            *do* interfere, the merge does not even start ('git read-tree'

            
            complains loudly and fails without modifying anything).  In such

            
            a case, you can simply continue doing what you were in the

            
            middle of doing, and when your working tree is ready (i.e. you

            
            have finished your work-in-progress), attempt the merge again.


            SPARSE CHECKOUT

            
            ---------------


            Note: The skip-worktree capabilities in linkgit:git-update-index[1]

            
            and `read-tree` predated the introduction of

            
            linkgit:git-sparse-checkout[1].  Users are encouraged to use the

            
            `sparse-checkout` command in preference to these plumbing commands for

            
            sparse-checkout/skip-worktree related needs.  However, the information

            
            below might be useful to users trying to understand the pattern style

            
            used in non-cone mode of the `sparse-checkout` command.


            "Sparse checkout" allows populating the working directory sparsely.

            
            It uses the skip-worktree bit (see linkgit:git-update-index[1]) to

            
            tell Git whether a file in the working directory is worth looking at.


            'git read-tree' and other merge-based commands ('git merge', 'git

            
            checkout'...) can help maintaining the skip-worktree bitmap and working

            
            directory update. `$GIT_DIR/info/sparse-checkout` is used to

            
            define the skip-worktree reference bitmap. When 'git read-tree' needs

            
            to update the working directory, it resets the skip-worktree bit in the index

            
            based on this file, which uses the same syntax as .gitignore files.

            
            If an entry matches a pattern in this file, or the entry corresponds to

            
            a file present in the working tree, then skip-worktree will not be

            
            set on that entry. Otherwise, skip-worktree will be set.


            Then it compares the new skip-worktree value with the previous one. If

            
            skip-worktree turns from set to unset, it will add the corresponding

            
            file back. If it turns from unset to set, that file will be removed.


            While `$GIT_DIR/info/sparse-checkout` is usually used to specify what

            
            files are in, you can also specify what files are _not_ in, using

            
            negate patterns. For example, to remove the file `unwanted`:


            ----------------

            
            /*

            
            !unwanted

            
            ----------------


            Another tricky thing is fully repopulating the working directory when you

            
            no longer want sparse checkout. You cannot just disable "sparse

            
            checkout" because skip-worktree bits are still in the index and your working

            
            directory is still sparsely populated. You should re-populate the working

            
            directory with the `$GIT_DIR/info/sparse-checkout` file content as

            
            follows:


            ----------------

            
            /*

            
            ----------------


            Then you can disable sparse checkout. Sparse checkout support in 'git

            
            read-tree' and similar commands is disabled by default. You need to

            
            turn `core.sparseCheckout` on in order to have sparse checkout

            
            support.


            SEE ALSO

            
            --------

            
            linkgit:git-write-tree[1], linkgit:git-ls-files[1],

            
            linkgit:gitignore[5], linkgit:git-sparse-checkout[1]


            GIT

            
            ---

            
            Part of the linkgit:git[1] suite
	git-read-tree(1)
	================

	NAME
	----
	git-read-tree - Reads tree information into the index


	SYNOPSIS
	--------
	[verse]
	'git read-tree' [(-m [--trivial] [--aggressive] \| --reset \| --prefix=<prefix>)
	[-u \| -i]] [--index-output=<file>] [--no-sparse-checkout]
	(--empty \| <tree-ish1> [<tree-ish2> [<tree-ish3>]])


	DESCRIPTION
	-----------
	Reads the tree information given by <tree-ish> into the index,
	but does not actually update any of the files it "caches". (see:
	linkgit:git-checkout-index[1])

	Optionally, it can merge a tree into the index, perform a
	fast-forward (i.e. 2-way) merge, or a 3-way merge, with the `-m`
	flag. When used with `-m`, the `-u` flag causes it to also update
	the files in the work tree with the result of the merge.

	Only trivial merges are done by 'git read-tree' itself. Only conflicting paths
	will be in an unmerged state when 'git read-tree' returns.

	OPTIONS
	-------
	-m::
	Perform a merge, not just a read. The command will
	refuse to run if your index file has unmerged entries,
	indicating that you have not finished a previous merge you
	started.

	--reset::
	Same as -m, except that unmerged entries are discarded instead
	of failing. When used with `-u`, updates leading to loss of
	working tree changes or untracked files or directories will not
	abort the operation.

	-u::
	After a successful merge, update the files in the work
	tree with the result of the merge.

	-i::
	Usually a merge requires the index file as well as the
	files in the working tree to be up to date with the
	current head commit, in order not to lose local
	changes. This flag disables the check with the working
	tree and is meant to be used when creating a merge of
	trees that are not directly related to the current
	working tree status into a temporary index file.

	-n::
	--dry-run::
	Check if the command would error out, without updating the index
	or the files in the working tree for real.

	-v::
	Show the progress of checking files out.

	--trivial::
	Restrict three-way merge by 'git read-tree' to happen
	only if there is no file-level merging required, instead
	of resolving merge for trivial cases and leaving
	conflicting files unresolved in the index.

	--aggressive::
	Usually a three-way merge by 'git read-tree' resolves
	the merge for really trivial cases and leaves other
	cases unresolved in the index, so that porcelains can
	implement different merge policies. This flag makes the
	command resolve a few more cases internally:
	+
	* when one side removes a path and the other side leaves the path
	unmodified. The resolution is to remove that path.
	* when both sides remove a path. The resolution is to remove that path.
	* when both sides add a path identically. The resolution
	is to add that path.

	--prefix=<prefix>::
	Keep the current index contents, and read the contents
	of the named tree-ish under the directory at `<prefix>`.
	The command will refuse to overwrite entries that already
	existed in the original index file.

	--index-output=<file>::
	Instead of writing the results out to `$GIT_INDEX_FILE`,
	write the resulting index in the named file. While the
	command is operating, the original index file is locked
	with the same mechanism as usual. The file must allow
	to be rename(2)ed into from a temporary file that is
	created next to the usual index file; typically this
	means it needs to be on the same filesystem as the index
	file itself, and you need write permission to the
	directories the index file and index output file are
	located in.

	--recurse-submodules::
	--no-recurse-submodules::
	Using --recurse-submodules will update the content of all active
	submodules according to the commit recorded in the superproject by
	calling read-tree recursively, also setting the submodules' HEAD to be
	detached at that commit.

	--no-sparse-checkout::
	Disable sparse checkout support even if `core.sparseCheckout`
	is true.

	--empty::
	Instead of reading tree object(s) into the index, just empty
	it.

	-q::
	--quiet::
	Quiet, suppress feedback messages.

	<tree-ish#>::
	The id of the tree object(s) to be read/merged.


	MERGING
	-------
	If `-m` is specified, 'git read-tree' can perform 3 kinds of
	merge, a single tree merge if only 1 tree is given, a
	fast-forward merge with 2 trees, or a 3-way merge if 3 or more trees are
	provided.


	Single Tree Merge
	~~~~~~~~~~~~~~~~~
	If only 1 tree is specified, 'git read-tree' operates as if the user did not
	specify `-m`, except that if the original index has an entry for a
	given pathname, and the contents of the path match with the tree
	being read, the stat info from the index is used. (In other words, the
	index's stat()s take precedence over the merged tree's).

	That means that if you do a `git read-tree -m <newtree>` followed by a
	`git checkout-index -f -u -a`, the 'git checkout-index' only checks out
	the stuff that really changed.

	This is used to avoid unnecessary false hits when 'git diff-files' is
	run after 'git read-tree'.


	Two Tree Merge
	~~~~~~~~~~~~~~

	Typically, this is invoked as `git read-tree -m $H $M`, where $H
	is the head commit of the current repository, and $M is the head
	of a foreign tree, which is simply ahead of $H (i.e. we are in a
	fast-forward situation).

	When two trees are specified, the user is telling 'git read-tree'
	the following:

	1. The current index and work tree is derived from $H, but
	the user may have local changes in them since $H.

	2. The user wants to fast-forward to $M.

	In this case, the `git read-tree -m $H $M` command makes sure
	that no local change is lost as the result of this "merge".
	Here are the "carry forward" rules, where "I" denotes the index,
	"clean" means that index and work tree coincide, and "exists"/"nothing"
	refer to the presence of a path in the specified commit:

	....
	I H M Result
	-------------------------------------------------------
	0 nothing nothing nothing (does not happen)
	1 nothing nothing exists use M
	2 nothing exists nothing remove path from index
	3 nothing exists exists, use M if "initial checkout",
	H == M keep index otherwise
	exists, fail
	H != M

	clean I==H I==M
	------------------
	4 yes N/A N/A nothing nothing keep index
	5 no N/A N/A nothing nothing keep index

	6 yes N/A yes nothing exists keep index
	7 no N/A yes nothing exists keep index
	8 yes N/A no nothing exists fail
	9 no N/A no nothing exists fail

	10 yes yes N/A exists nothing remove path from index
	11 no yes N/A exists nothing fail
	12 yes no N/A exists nothing fail
	13 no no N/A exists nothing fail

	clean (H==M)
	------
	14 yes exists exists keep index
	15 no exists exists keep index

	clean I==H I==M (H!=M)
	------------------
	16 yes no no exists exists fail
	17 no no no exists exists fail
	18 yes no yes exists exists keep index
	19 no no yes exists exists keep index
	20 yes yes no exists exists use M
	21 no yes no exists exists fail
	....

	In all "keep index" cases, the index entry stays as in the
	original index file. If the entry is not up to date,
	'git read-tree' keeps the copy in the work tree intact when
	operating under the -u flag.

	When this form of 'git read-tree' returns successfully, you can
	see which of the "local changes" that you made were carried forward by running
	`git diff-index --cached $M`. Note that this does not
	necessarily match what `git diff-index --cached $H` would have
	produced before such a two tree merge. This is because of cases
	18 and 19 -- if you already had the changes in $M (e.g. maybe
	you picked it up via e-mail in a patch form), `git diff-index
	--cached $H` would have told you about the change before this
	merge, but it would not show in `git diff-index --cached $M`
	output after the two-tree merge.

	Case 3 is slightly tricky and needs explanation. The result from this
	rule logically should be to remove the path if the user staged the removal
	of the path and then switching to a new branch. That however will prevent
	the initial checkout from happening, so the rule is modified to use M (new
	tree) only when the content of the index is empty. Otherwise the removal
	of the path is kept as long as $H and $M are the same.

	3-Way Merge
	~~~~~~~~~~~
	Each "index" entry has two bits worth of "stage" state. stage 0 is the
	normal one, and is the only one you'd see in any kind of normal use.

	However, when you do 'git read-tree' with three trees, the "stage"
	starts out at 1.

	This means that you can do

	----------------
	$ git read-tree -m <tree1> <tree2> <tree3>
	----------------

	and you will end up with an index with all of the <tree1> entries in
	"stage1", all of the <tree2> entries in "stage2" and all of the
	<tree3> entries in "stage3". When performing a merge of another
	branch into the current branch, we use the common ancestor tree
	as <tree1>, the current branch head as <tree2>, and the other
	branch head as <tree3>.

	Furthermore, 'git read-tree' has special-case logic that says: if you see
	a file that matches in all respects in the following states, it
	"collapses" back to "stage0":

	- stage 2 and 3 are the same; take one or the other (it makes no
	difference - the same work has been done on our branch in
	stage 2 and their branch in stage 3)

	- stage 1 and stage 2 are the same and stage 3 is different; take
	stage 3 (our branch in stage 2 did not do anything since the
	ancestor in stage 1 while their branch in stage 3 worked on
	it)

	- stage 1 and stage 3 are the same and stage 2 is different take
	stage 2 (we did something while they did nothing)

	The 'git write-tree' command refuses to write a nonsensical tree, and it
	will complain about unmerged entries if it sees a single entry that is not
	stage 0.

	OK, this all sounds like a collection of totally nonsensical rules,
	but it's actually exactly what you want in order to do a fast
	merge. The different stages represent the "result tree" (stage 0, aka
	"merged"), the original tree (stage 1, aka "orig"), and the two trees
	you are trying to merge (stage 2 and 3 respectively).

	The order of stages 1, 2 and 3 (hence the order of three
	<tree-ish> command-line arguments) are significant when you
	start a 3-way merge with an index file that is already
	populated. Here is an outline of how the algorithm works:

	- if a file exists in identical format in all three trees, it will
	automatically collapse to "merged" state by 'git read-tree'.

	- a file that has _any_ difference what-so-ever in the three trees
	will stay as separate entries in the index. It's up to "porcelain
	policy" to determine how to remove the non-0 stages, and insert a
	merged version.

	- the index file saves and restores with all this information, so you
	can merge things incrementally, but as long as it has entries in
	stages 1/2/3 (i.e., "unmerged entries") you can't write the result. So
	now the merge algorithm ends up being really simple:

	* you walk the index in order, and ignore all entries of stage 0,
	since they've already been done.

	* if you find a "stage1", but no matching "stage2" or "stage3", you
	know it's been removed from both trees (it only existed in the
	original tree), and you remove that entry.

	* if you find a matching "stage2" and "stage3" tree, you remove one
	of them, and turn the other into a "stage0" entry. Remove any
	matching "stage1" entry if it exists too. .. all the normal
	trivial rules ..

	You would normally use 'git merge-index' with supplied
	'git merge-one-file' to do this last step. The script updates
	the files in the working tree as it merges each path and at the
	end of a successful merge.

	When you start a 3-way merge with an index file that is already
	populated, it is assumed that it represents the state of the
	files in your work tree, and you can even have files with
	changes unrecorded in the index file. It is further assumed
	that this state is "derived" from the stage 2 tree. The 3-way
	merge refuses to run if it finds an entry in the original index
	file that does not match stage 2.

	This is done to prevent you from losing your work-in-progress
	changes, and mixing your random changes in an unrelated merge
	commit. To illustrate, suppose you start from what has been
	committed last to your repository:

	----------------
	$ JC=`git rev-parse --verify "HEAD^0"`
	$ git checkout-index -f -u -a $JC
	----------------

	You do random edits, without running 'git update-index'. And then
	you notice that the tip of your "upstream" tree has advanced
	since you pulled from him:

	----------------
	$ git fetch git://.... linus
	$ LT=`git rev-parse FETCH_HEAD`
	----------------

	Your work tree is still based on your HEAD ($JC), but you have
	some edits since. Three-way merge makes sure that you have not
	added or modified index entries since $JC, and if you haven't,
	then does the right thing. So with the following sequence:

	----------------
	$ git read-tree -m -u `git merge-base $JC $LT` $JC $LT
	$ git merge-index git-merge-one-file -a
	$ echo "Merge with Linus" \| \
	git commit-tree `git write-tree` -p $JC -p $LT
	----------------

	what you would commit is a pure merge between $JC and $LT without
	your work-in-progress changes, and your work tree would be
	updated to the result of the merge.

	However, if you have local changes in the working tree that
	would be overwritten by this merge, 'git read-tree' will refuse
	to run to prevent your changes from being lost.

	In other words, there is no need to worry about what exists only
	in the working tree. When you have local changes in a part of
	the project that is not involved in the merge, your changes do
	not interfere with the merge, and are kept intact. When they
	do interfere, the merge does not even start ('git read-tree'
	complains loudly and fails without modifying anything). In such
	a case, you can simply continue doing what you were in the
	middle of doing, and when your working tree is ready (i.e. you
	have finished your work-in-progress), attempt the merge again.


	SPARSE CHECKOUT
	---------------

	Note: The skip-worktree capabilities in linkgit:git-update-index[1]
	and `read-tree` predated the introduction of
	linkgit:git-sparse-checkout[1]. Users are encouraged to use the
	`sparse-checkout` command in preference to these plumbing commands for
	sparse-checkout/skip-worktree related needs. However, the information
	below might be useful to users trying to understand the pattern style
	used in non-cone mode of the `sparse-checkout` command.

	"Sparse checkout" allows populating the working directory sparsely.
	It uses the skip-worktree bit (see linkgit:git-update-index[1]) to
	tell Git whether a file in the working directory is worth looking at.

	'git read-tree' and other merge-based commands ('git merge', 'git
	checkout'...) can help maintaining the skip-worktree bitmap and working
	directory update. `$GIT_DIR/info/sparse-checkout` is used to
	define the skip-worktree reference bitmap. When 'git read-tree' needs
	to update the working directory, it resets the skip-worktree bit in the index
	based on this file, which uses the same syntax as .gitignore files.
	If an entry matches a pattern in this file, or the entry corresponds to
	a file present in the working tree, then skip-worktree will not be
	set on that entry. Otherwise, skip-worktree will be set.

	Then it compares the new skip-worktree value with the previous one. If
	skip-worktree turns from set to unset, it will add the corresponding
	file back. If it turns from unset to set, that file will be removed.

	While `$GIT_DIR/info/sparse-checkout` is usually used to specify what
	files are in, you can also specify what files are _not_ in, using
	negate patterns. For example, to remove the file `unwanted`:

	----------------
	/*
	!unwanted
	----------------

	Another tricky thing is fully repopulating the working directory when you
	no longer want sparse checkout. You cannot just disable "sparse
	checkout" because skip-worktree bits are still in the index and your working
	directory is still sparsely populated. You should re-populate the working
	directory with the `$GIT_DIR/info/sparse-checkout` file content as
	follows:

	----------------
	/*
	----------------

	Then you can disable sparse checkout. Sparse checkout support in 'git
	read-tree' and similar commands is disabled by default. You need to
	turn `core.sparseCheckout` on in order to have sparse checkout
	support.


	SEE ALSO
	--------
	linkgit:git-write-tree[1], linkgit:git-ls-files[1],
	linkgit:gitignore[5], linkgit:git-sparse-checkout[1]

	GIT
	---
	Part of the linkgit:git[1] suite