replace: mark unused parameter in each_mergetag_fn callback

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>