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/verify-tag.c

@@ -72,7 +72,7 @@ int cmd_verify_tag(int argc, const char **argv, const char *prefix)
 		}
 
 		if (format.format)
-			pretty_print_ref(name, oid.hash, &format);
+			pretty_print_ref(name, &oid, &format);
 	}
 	return had_error;
 }