Convert lookup_tag to struct object_id

Convert lookup_tag to take a pointer to struct object_id.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/replace.c b/builtin/replace.c
index 3c44ef7..c921bc9 100644
--- a/builtin/replace.c
+++ b/builtin/replace.c
@@ -355,7 +355,7 @@ static void check_one_mergetag(struct commit *commit,
 	int i;
 
 	hash_sha1_file(extra->value, extra->len, typename(OBJ_TAG), tag_oid.hash);
-	tag = lookup_tag(tag_oid.hash);
+	tag = lookup_tag(&tag_oid);
 	if (!tag)
 		die(_("bad mergetag in commit '%s'"), ref);
 	if (parse_tag_buffer(tag, extra->value, extra->len))