replace: mark unused parameter in each_mergetag_fn callback

We don't look at the "commit" parameter to our callback, as our "mergetag_data" pointer contains the original name "ref", which we use instead. But we can't get rid of it, since other for_each_mergetag callbacks do use it. Let's mark the parameter to avoid -Wunused-parameter warnings. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>