do_for_each_ref_helper(): mark unused repository parameter

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>