builtin-merge: release the lockfile in try_merge_strategy()

Once we committed the locked index, we should release the lockfile. In
most cases this is done automatically when the process ends, but this is
not true in this case.

[jc: with additional tests from Eric Raible]
Signed-off-by: NMiklos Vajna <vmiklos@frugalware.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin-merge.c

@@ -576,6 +576,7 @@ static int try_merge_strategy(const char *strategy, struct commit_list *common,
 				(write_cache(index_fd, active_cache, active_nr) ||
 				 commit_locked_index(lock)))
 			die ("unable to write %s", get_index_file());
+		rollback_lock_file(lock);
 		return clean ? 0 : 1;
 	} else {
 		args = xmalloc((4 + commit_list_count(common) +

t/t6026-merge-attr.sh

@@ -142,4 +142,26 @@ test_expect_success 'custom merge backend' '
 	rm -f $o $a $b
 '
 
+test_expect_success 'up-to-date merge without common ancestor' '
+	test_create_repo repo1 &&
+	test_create_repo repo2 &&
+	test_tick &&
+	(
+		cd repo1 &&
+		>a &&
+		git add a &&
+		git commit -m initial
+	) &&
+	test_tick &&
+	(
+		cd repo2 &&
+		git commit --allow-empty -m initial
+	) &&
+	test_tick &&
+	(
+		cd repo1 &&
+		git pull ../repo2 master
+	)
+'
+
 test_done