t/helper: merge (unused) test-mergesort into test-tool

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

t/helper/test-tool.c

@@ -21,6 +21,7 @@ static struct test_cmd cmds[] = {
 	{ "index-version", cmd__index_version },
 	{ "lazy-init-name-hash", cmd__lazy_init_name_hash },
 	{ "match-trees", cmd__match_trees },
+	{ "mergesort", cmd__mergesort },
 	{ "sha1", cmd__sha1 },
 };