pack-bitmap: implement test helpers for pseudo-merge

pack-bitmap: implement test helpers for pseudo-merge

Implement three new sub-commands for the "bitmap" test-helper:

  - t/helper test-tool bitmap dump-pseudo-merges
  - t/helper test-tool bitmap dump-pseudo-merge-commits <n>
  - t/helper test-tool bitmap dump-pseudo-merge-objects <n>

These three helpers dump the list of pseudo merges, the "parents" of the
nth pseudo-merges, and the set of objects reachable from those parents,
respectively.

These helpers will be useful in subsequent patches when we add test
coverage for pseudo-merge bitmaps.

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