pack-bitmap: save "have" bitmap from walk

pack-bitmap: save "have" bitmap from walk

When we do a bitmap walk, we save the result, which
represents (WANTs & ~HAVEs); i.e., every object we care
about visiting in our walk. However, we throw away the
haves bitmap, which can sometimes be useful, too. Save it
and provide an access function so code which has performed a
walk can query it.

A few notes on the accessor interface:

 - the bitmap code calls these "haves" because it grew out
   of the want/have negotiation for fetches. But really,
   these are simply the objects that would be flagged
   UNINTERESTING in a regular traversal. Let's use that
   more universal nomenclature for the external module
   interface. We may want to change the internal naming
   inside the bitmap code, but that's outside the scope of
   this patch.

 - it still uses a bare "sha1" rather than "oid". That's
   true of all of the bitmap code. And in this particular
   instance, our caller in pack-objects is dealing with the
   bare sha1 that comes from a packed REF_DELTA (we're
   pointing directly to the mmap'd pack on disk). That's
   something we'll have to deal with as we transition to a
   new hash, but we can wait and see how the caller ends up
   being fixed and adjust this interface accordingly.

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