rev-list: use bitmap filters for traversal

rev-list: use bitmap filters for traversal

This just passes the filter-options struct to prepare_bitmap_walk().
Since the bitmap code doesn't actually support any filters yet, it will
fallback to the non-bitmap code if any --filter is specified. But this
lets us exercise that rejection code path, as well as getting us ready
to test filters via rev-list when we _do_ support them.

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