ref-filter: use "struct object_id" consistently

Internally we store a "struct object_id", and all of our
callers have one to pass us. But we insist that they peel it
to its bare-sha1 hash, which we then hashcpy() into place.
Let's pass it around as an object_id, which future-proofs us
for a post-sha1 world.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/tag.c

@@ -117,7 +117,7 @@ static int verify_tag(const char *name, const char *ref,
 		return -1;
 
 	if (format->format)
-		pretty_print_ref(name, oid->hash, format);
+		pretty_print_ref(name, oid, format);
 
 	return 0;
 }