pack-objects: support filters with bitmaps

pack-objects: support filters with bitmaps

Just as rev-list recently learned to combine filters and bitmaps, let's
do the same for pack-objects. The infrastructure is all there; we just
need to pass along our filter options, and the pack-bitmap code will
decide to use bitmaps or not.

This unsurprisingly makes things faster for partial clones of large
repositories (here we're cloning linux.git):

  Test                               HEAD^               HEAD
  ------------------------------------------------------------------------------
  5310.11: simulated partial clone   38.94(37.28+5.87)   11.06(11.27+4.07) -71.6%

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