Provide 'git merge --abort' as a synonym to 'git reset --merge'

Teach 'git merge' the --abort option, which verifies the existence of
MERGE_HEAD and then invokes 'git reset --merge' to abort the current
in-progress merge and attempt to reconstruct the pre-merge state.

The reason for adding this option is to provide a user interface for
aborting an in-progress merge that is consistent with the interface
for aborting a rebase ('git rebase --abort'), aborting the application
of a patch series ('git am --abort'), and aborting an in-progress notes
merge ('git notes merge --abort').

The patch includes documentation and testcases that explain and verify
the various scenarios in which 'git merge --abort' can run. The
testcases also document the cases in which 'git merge --abort' is
unable to correctly restore the pre-merge state (look for the '###'
comments towards the bottom of t/t7609-merge-abort.sh).

This patch has been improved by the following contributions:
- Jonathan Nieder: Move test documentation into test_description

Thanks-to: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-merge.txt

@@ -13,6 +13,7 @@ SYNOPSIS
 	[-s <strategy>] [-X <strategy-option>]
 	[--[no-]rerere-autoupdate] [-m <msg>] <commit>...
 'git merge' <msg> HEAD <commit>...
+'git merge' --abort
 
 DESCRIPTION
 -----------
@@ -47,6 +48,14 @@ The second syntax (<msg> `HEAD` <commit>...) is supported for
 historical reasons.  Do not use it from the command line or in
 new scripts.  It is the same as `git merge -m <msg> <commit>...`.
 
+The third syntax ("`git merge --abort`") can only be run after the
+merge has resulted in conflicts. 'git merge --abort' will abort the
+merge process and try to reconstruct the pre-merge state. However,
+if there were uncommitted changes when the merge started (and
+especially if those changes were further modified after the merge
+was started), 'git merge --abort' will in some cases be unable to
+reconstruct the original (pre-merge) changes. Therefore:
+
 *Warning*: Running 'git merge' with uncommitted changes is
 discouraged: while possible, it leaves you in a state that is hard to
 back out of in the case of a conflict.
@@ -72,6 +81,18 @@ include::merge-options.txt[]
 	Allow the rerere mechanism to update the index with the
 	result of auto-conflict resolution if possible.
 
+--abort::
+	Abort the current conflict resolution process, and
+	try to reconstruct the pre-merge state.
++
+If there were uncommitted worktree changes present when the merge
+started, 'git merge --abort' will in some cases be unable to
+reconstruct these changes. It is therefore recommended to always
+commit or stash your changes before running 'git merge'.
++
+'git merge --abort' is equivalent to 'git reset --merge' when
+`MERGE_HEAD` is present.
+
 <commit>...::
 	Commits, usually other branch heads, to merge into our branch.
 	You need at least one <commit>.  Specifying more than one
@@ -142,7 +163,7 @@ happens:
    i.e. matching `HEAD`.
 
 If you tried a merge which resulted in complex conflicts and
-want to start over, you can recover with `git reset --merge`.
+want to start over, you can recover with `git merge --abort`.
 
 HOW CONFLICTS ARE PRESENTED
 ---------------------------
@@ -213,8 +234,8 @@ After seeing a conflict, you can do two things:
 
  * Decide not to merge.  The only clean-ups you need are to reset
    the index file to the `HEAD` commit to reverse 2. and to clean
-   up working tree changes made by 2. and 3.; `git-reset --hard` can
-   be used for this.
+   up working tree changes made by 2. and 3.; `git merge --abort`
+   can be used for this.
 
  * Resolve the conflicts.  Git will mark the conflicts in
    the working tree.  Edit the files into shape and

builtin/merge.c

@@ -56,6 +56,7 @@ static size_t xopts_nr, xopts_alloc;
 static const char *branch;
 static int verbosity;
 static int allow_rerere_auto;
+static int abort_current_merge;
 
 static struct strategy all_strategy[] = {
 	{ "recursive",  DEFAULT_TWOHEAD | NO_TRIVIAL },
@@ -194,6 +195,8 @@ static struct option builtin_merge_options[] = {
 		"message to be used for the merge commit (if any)",
 		option_parse_message),
 	OPT__VERBOSITY(&verbosity),
+	OPT_BOOLEAN(0, "abort", &abort_current_merge,
+		"abort the current in-progress merge"),
 	OPT_END()
 };
 
@@ -914,6 +917,17 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 	argc = parse_options(argc, argv, prefix, builtin_merge_options,
 			builtin_merge_usage, 0);
 
+	if (abort_current_merge) {
+		int nargc = 2;
+		const char *nargv[] = {"reset", "--merge", NULL};
+
+		if (!file_exists(git_path("MERGE_HEAD")))
+			die("There is no merge to abort (MERGE_HEAD missing).");
+
+		/* Invoke 'git reset --merge' */
+		return cmd_reset(nargc, nargv, prefix);
+	}
+
 	if (read_cache_unmerged())
 		die_resolve_conflict("merge");