Generating patch text with -p
-----------------------------

Running
linkgit:git-diff[1],
linkgit:git-log[1],
linkgit:git-show[1],
linkgit:git-diff-index[1],
linkgit:git-diff-tree[1], or
linkgit:git-diff-files[1]
with the `-p` option produces patch text.
You can customize the creation of patch text via the
`GIT_EXTERNAL_DIFF` and the `GIT_DIFF_OPTS` environment variables.

What the -p option produces is slightly different from the traditional
diff format:

1.   It is preceded with a "git diff" header that looks like this:

       diff --git a/file1 b/file2
+
The `a/` and `b/` filenames are the same unless rename/copy is
involved.  Especially, even for a creation or a deletion,
`/dev/null` is _not_ used in place of the `a/` or `b/` filenames.
+
When rename/copy is involved, `file1` and `file2` show the
name of the source file of the rename/copy and the name of
the file that rename/copy produces, respectively.

2.   It is followed by one or more extended header lines:

       old mode <mode>
       new mode <mode>
       deleted file mode <mode>
       new file mode <mode>
       copy from <path>
       copy to <path>
       rename from <path>
       rename to <path>
       similarity index <number>
       dissimilarity index <number>
       index <hash>..<hash> <mode>
+
File modes are printed as 6-digit octal numbers including the file type
and file permission bits.
+
Path names in extended headers do not include the `a/` and `b/` prefixes.
+
The similarity index is the percentage of unchanged lines, and
the dissimilarity index is the percentage of changed lines.  It
is a rounded down integer, followed by a percent sign.  The
similarity index value of 100% is thus reserved for two equal
files, while 100% dissimilarity means that no line from the old
file made it into the new one.
+
The index line includes the blob object names before and after the change.
The <mode> is included if the file mode does not change; otherwise,
separate lines indicate the old and the new mode.

3.  Pathnames with "unusual" characters are quoted as explained for
    the configuration variable `core.quotePath` (see
    linkgit:git-config[1]).

4.  All the `file1` files in the output refer to files before the
    commit, and all the `file2` files refer to files after the commit.
    It is incorrect to apply each change to each file sequentially.  For
    example, this patch will swap a and b:

      diff --git a/a b/b
      rename from a
      rename to b
      diff --git a/b b/a
      rename from b
      rename to a


Combined diff format
--------------------

Any diff-generating command can take the `-c` or `--cc` option to
produce a 'combined diff' when showing a merge. This is the default
format when showing merges with linkgit:git-diff[1] or
linkgit:git-show[1]. Note also that you can give the `-m` option to any
of these commands to force generation of diffs with individual parents
of a merge.

A "combined diff" format looks like this:

------------
diff --combined describe.c
index fabadb8,cc95eb0..4866510
--- a/describe.c
+++ b/describe.c
@@@ -98,20 -98,12 +98,20 @@@
	return (a_date > b_date) ? -1 : (a_date == b_date) ? 0 : 1;
  }

- static void describe(char *arg)
 -static void describe(struct commit *cmit, int last_one)
++static void describe(char *arg, int last_one)
  {
 +	unsigned char sha1[20];
 +	struct commit *cmit;
	struct commit_list *list;
	static int initialized = 0;
	struct commit_name *n;

 +	if (get_sha1(arg, sha1) < 0)
 +		usage(describe_usage);
 +	cmit = lookup_commit_reference(sha1);
 +	if (!cmit)
 +		usage(describe_usage);
 +
	if (!initialized) {
		initialized = 1;
		for_each_ref(get_name);
------------

1.   It is preceded with a "git diff" header, that looks like
     this (when the `-c` option is used):

       diff --combined file
+
or like this (when the `--cc` option is used):

       diff --cc file

2.   It is followed by one or more extended header lines
     (this example shows a merge with two parents):

       index <hash>,<hash>..<hash>
       mode <mode>,<mode>..<mode>
       new file mode <mode>
       deleted file mode <mode>,<mode>
+
The `mode <mode>,<mode>..<mode>` line appears only if at least one of
the <mode> is different from the rest. Extended headers with
information about detected contents movement (renames and
copying detection) are designed to work with diff of two
<tree-ish> and are not used by combined diff format.

3.   It is followed by two-line from-file/to-file header

       --- a/file
       +++ b/file
+
Similar to two-line header for traditional 'unified' diff
format, `/dev/null` is used to signal created or deleted
files.
+
However, if the --combined-all-paths option is provided, instead of a
two-line from-file/to-file you get a N+1 line from-file/to-file header,
where N is the number of parents in the merge commit

       --- a/file
       --- a/file
       --- a/file
       +++ b/file
+
This extended format can be useful if rename or copy detection is
active, to allow you to see the original name of the file in different
parents.

4.   Chunk header format is modified to prevent people from
     accidentally feeding it to `patch -p1`. Combined diff format
     was created for review of merge commit changes, and was not
     meant to be applied. The change is similar to the change in the
     extended 'index' header:

       @@@ <from-file-range> <from-file-range> <to-file-range> @@@
+
There are (number of parents + 1) `@` characters in the chunk
header for combined diff format.

Unlike the traditional 'unified' diff format, which shows two
files A and B with a single column that has `-` (minus --
appears in A but removed in B), `+` (plus -- missing in A but
added to B), or `" "` (space -- unchanged) prefix, this format
compares two or more files file1, file2,... with one file X, and
shows how X differs from each of fileN.  One column for each of
fileN is prepended to the output line to note how X's line is
different from it.

A `-` character in the column N means that the line appears in
fileN but it does not appear in the result.  A `+` character
in the column N means that the line appears in the result,
and fileN does not have that line (in other words, the line was
added, from the point of view of that parent).

In the above example output, the function signature was changed
from both files (hence two `-` removals from both file1 and
file2, plus `++` to mean one line that was added does not appear
in either file1 or file2).  Also eight other lines are the same
from file1 but do not appear in file2 (hence prefixed with `+`).

When shown by `git diff-tree -c`, it compares the parents of a
merge commit with the merge result (i.e. file1..fileN are the
parents).  When shown by `git diff-files -c`, it compares the
two unresolved merge parents with the working tree file
(i.e. file1 is stage 2 aka "our version", file2 is stage 3 aka
"their version").