do_for_each_ref_helper(): mark unused repository parameter

This function gets a repository parameter because it's a callback for do_for_each_repo_ref_iterator(). But it's just a wrapper that passes along each call to a regular each_ref_fn callback, and the latter doesn't accept a repository argument. Probably in the long run all of the each_ref_fn callbacks should get a repository parameter, too. But changing that now would require updates all over the code base. Until that happens, let's annotate this wrapper callback to quiet the compiler's -Wunused-parameter warning. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>