pack-bitmap: add 'test_bitmap_commits()' helper

pack-bitmap: add 'test_bitmap_commits()' helper

The next patch will add a 'bitmap' test-tool which prints the list of
commits that have bitmaps computed.

The test helper could implement this itself, but it would need access to
the 'bitmaps' field of the 'pack_bitmap' struct. To avoid exposing this
private detail, implement the entirety of the helper behind a
test_bitmap_commits() function in pack-bitmap.c.

There is some precedence for this with test_bitmap_walk() which is used
to implement the '--test-bitmap' flag in 'git rev-list' (and is also
implemented in pack-bitmap.c).

A caller will be added in the next patch.

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