revert.c: defer writing CHERRY_PICK_HEAD till it is safe to do so

do_pick_commit() writes out CHERRY_PICK_HEAD before invoking merge (either
via do_recursive_merge() or try_merge_command()) on the assumption that if
the merge fails it is due to conflict. However, if the tree is dirty, the
merge may not even start, aborting before do_pick_commit() can remove
CHERRY_PICK_HEAD.

Instead, defer writing CHERRY_PICK_HEAD till after merge has returned.
At this point we know the merge has either succeeded or failed due
to conflict. In either case, we want CHERRY_PICK_HEAD to be written
so that it may be picked up by the subsequent invocation of commit.

Note that do_recursive_merge() aborts if the merge cannot start, while
try_merge_command() returns a non-zero value other than 1.

Signed-off-by: Jay Soffian <jaysoffian@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/revert.c

@@ -481,8 +481,6 @@ static int do_pick_commit(void)
 			strbuf_addstr(&msgbuf, sha1_to_hex(commit->object.sha1));
 			strbuf_addstr(&msgbuf, ")\n");
 		}
-		if (!no_commit)
-			write_cherry_pick_head();
 	}
 
 	if (!strategy || !strcmp(strategy, "recursive") || action == REVERT) {
@@ -503,6 +501,15 @@ static int do_pick_commit(void)
 		free_commit_list(remotes);
 	}
 
+	/*
+	 * If the merge was clean or if it failed due to conflict, we write
+	 * CHERRY_PICK_HEAD for the subsequent invocation of commit to use.
+	 * However, if the merge did not even start, then we don't want to
+	 * write it at all.
+	 */
+	if (action == CHERRY_PICK && !no_commit && (res == 0 || res == 1))
+		write_cherry_pick_head();
+
 	if (res) {
 		error(action == REVERT
 		      ? _("could not revert %s... %s")

t/t3507-cherry-pick-conflict.sh

@@ -77,6 +77,21 @@ test_expect_success 'cherry-pick --no-commit does not set CHERRY_PICK_HEAD' '
 	test_must_fail git rev-parse --verify CHERRY_PICK_HEAD
 '
 
+test_expect_success 'cherry-pick w/dirty tree does not set CHERRY_PICK_HEAD' '
+	pristine_detach initial &&
+	echo foo > foo &&
+	test_must_fail git cherry-pick base &&
+	test_must_fail git rev-parse --verify CHERRY_PICK_HEAD
+'
+
+test_expect_success \
+	'cherry-pick --strategy=resolve w/dirty tree does not set CHERRY_PICK_HEAD' '
+	pristine_detach initial &&
+	echo foo > foo &&
+	test_must_fail git cherry-pick --strategy=resolve base &&
+	test_must_fail git rev-parse --verify CHERRY_PICK_HEAD
+'
+
 test_expect_success 'GIT_CHERRY_PICK_HELP suppresses CHERRY_PICK_HEAD' '
 	pristine_detach initial &&
 	(