merge: add --quit

This allows to cancel the current merge without resetting worktree/index,
which is what --abort is for. Like other --quit(s), this is often used
when you forgot that you're in the middle of a merge and already
switched away, doing different things. By the time you've realized, you
can't even continue the merge anymore.

This also makes all in-progress commands, am, merge, rebase, revert and
cherry-pick, take all three --abort, --continue and --quit (bisect has a
different UI).

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-merge.txt

diff --git a/Documentation/git-merge.txt b/Documentation/git-merge.txt
index 4cc8646..b7d581f 100644
--- a/Documentation/git-merge.txt
+++ b/Documentation/git-merge.txt
@@ -99,6 +99,10 @@
 'git merge --abort' is equivalent to 'git reset --merge' when
 `MERGE_HEAD` is present.
 
+--quit::
+	Forget about the current merge in progress. Leave the index
+	and the working tree as-is.
+
 --continue::
 	After a 'git merge' stops due to conflicts you can conclude the
 	merge by running 'git merge --continue' (see "HOW TO RESOLVE

builtin/merge.c

diff --git a/builtin/merge.c b/builtin/merge.c
index e9663f0..598d56e 100644
--- a/builtin/merge.c
+++ b/builtin/merge.c
@@ -73,6 +73,7 @@ static int option_renormalize;
 static int verbosity;
 static int allow_rerere_auto;
 static int abort_current_merge;
+static int quit_current_merge;
 static int continue_current_merge;
 static int allow_unrelated_histories;
 static int show_progress = -1;
@@ -267,6 +268,8 @@ static struct option builtin_merge_options[] = {
 	OPT__VERBOSITY(&verbosity),
 	OPT_BOOL(0, "abort", &abort_current_merge,
 		N_("abort the current in-progress merge")),
+	OPT_BOOL(0, "quit", &quit_current_merge,
+		N_("--abort but leave index and working tree alone")),
 	OPT_BOOL(0, "continue", &continue_current_merge,
 		N_("continue the current in-progress merge")),
 	OPT_BOOL(0, "allow-unrelated-histories", &allow_unrelated_histories,
@@ -1252,6 +1255,16 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 		goto done;
 	}
 
+	if (quit_current_merge) {
+		if (orig_argc != 2)
+			usage_msg_opt(_("--quit expects no arguments"),
+				      builtin_merge_usage,
+				      builtin_merge_options);
+
+		remove_merge_branch_state(the_repository);
+		goto done;
+	}
+
 	if (continue_current_merge) {
 		int nargc = 1;
 		const char *nargv[] = {"commit", NULL};

t/t7600-merge.sh

diff --git a/t/t7600-merge.sh b/t/t7600-merge.sh
index 1061482..625a24a 100755
--- a/t/t7600-merge.sh
+++ b/t/t7600-merge.sh
@@ -822,4 +822,30 @@
 	verify_parents $c0 $c1
 '
 
+test_expect_success 'merge --quit' '
+	git init merge-quit &&
+	(
+		cd merge-quit &&
+		test_commit base &&
+		echo one >>base.t &&
+		git commit -am one &&
+		git branch one &&
+		git checkout base &&
+		echo two >>base.t &&
+		git commit -am two &&
+		test_must_fail git -c rerere.enabled=true merge one &&
+		test_path_is_file .git/MERGE_HEAD &&
+		test_path_is_file .git/MERGE_MODE &&
+		test_path_is_file .git/MERGE_MSG &&
+		git rerere status >rerere.before &&
+		git merge --quit &&
+		test_path_is_missing .git/MERGE_HEAD &&
+		test_path_is_missing .git/MERGE_MODE &&
+		test_path_is_missing .git/MERGE_MSG &&
+		git rerere status >rerere.after &&
+		test_must_be_empty rerere.after &&
+		! test_cmp rerere.after rerere.before
+	)
+'
+
 test_done