list-objects: mark unused callback parameters

list-objects: mark unused callback parameters

Our graph-traversal functions take callbacks for showing commits and
objects, but not all callbacks need each parameter.  Likewise for the
similar traverse_bitmap_commit_list(), which has a different interface
but serves the same purpose. And the include_check mechanism, which
passes along a void pointer which is not always used.

Mark the unused ones to to make -Wunused-parameter happy.

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