pack-bitmap.c: support 'tree:0' filtering

pack-bitmap.c: support 'tree:0' filtering

In the previous patch, we made it easy to define other filters that
exclude all objects of a certain type. Use that in order to implement
bitmap-level filtering for the '--filter=tree:<n>' filter when 'n' is
equal to 0.

The general case is not helped by bitmaps, since for values of 'n > 0',
the object filtering machinery requires a full-blown tree traversal in
order to determine the depth of a given tree. Caching this is
non-obvious, too, since the same tree object can have a different depth
depending on the context (e.g., a tree was moved up in the directory
hierarchy between two commits).

But, the 'n = 0' case can be helped, and this patch does so. Running
p5310.11 in this tree and on master with the kernel, we can see that
this case is helped substantially:

  Test                                  master              this tree
  --------------------------------------------------------------------------------
  5310.11: rev-list count with tree:0   10.68(10.39+0.27)   0.06(0.04+0.01) -99.4%

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>