fetch: mark unused parameter in ref_transaction callback

Since this callback is just trying to collect the set of queued tag
updates, there is no need for it to look at old_oid at all. Mark it as
unused to appease -Wunused-parameter.

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

builtin/fetch.c

@@ -308,7 +308,7 @@ static void clear_item(struct refname_hash_entry *item)
 
 
 static void add_already_queued_tags(const char *refname,
-				    const struct object_id *old_oid,
+				    const struct object_id *old_oid UNUSED,
 				    const struct object_id *new_oid,
 				    void *cb_data)
 {