[PATCH 0/3] pack-objects: introduce pack.preferBitmapTips

This series adds a new multi-valued configuration, pack.preferBitmapTips to allow callers to specify a list of reference prefixes which should mark their tips with the NEEDS_BITMAP flag.

This has a similar effect as setting pack.islandCore: it indicates to the bitmap selection code that any commit object with that flag should get bitmap coverage (it doesn't necessarily imply that it *will*, see the third patch for details of why).

This is more fall-out from the multi-pack bitmaps topic. Since we generate MIDX bitmaps after a geometric repack, any delta-islands configuration only takes effect on objects in the newly-created pack. Any commits which weren't in that pack may not get the same coverage they otherwise would have if repacking all-into-one with the same delta-islands settings.

So, this config was designed to suggest which commits should get marked with NEEDS_BITMAP when the MIDX bitmap code does its own traversal to figure out which reachable objects are in the MIDX. But it's useful for single-pack bitmaps, too, if you're using them without delta islands.

(Like all of the MIDX bitmap topics, we have been running this code at GitHub without any issue for a few weeks.)

Thanks in advance for your review.

Taylor Blau (3):
  pack-bitmap: add 'test_bitmap_commits()' helper
  t/helper/test-bitmap.c: initial commit
  builtin/pack-objects.c: respect 'pack.preferBitmapTips'

 Documentation/config/pack.txt | 15 ++++++++++++++
 Makefile                      |  1 +
 builtin/pack-objects.c        | 34 +++++++++++++++++++++++++++++++
 pack-bitmap.c                 | 24 ++++++++++++++++++++++
 pack-bitmap.h                 |  4 ++++
 t/helper/test-bitmap.c        | 24 ++++++++++++++++++++++
 t/helper/test-tool.c          |  1 +
 t/helper/test-tool.h          |  1 +
 t/t5310-pack-bitmaps.sh       | 38 +++++++++++++++++++++++++++++++++++
 9 files changed, 142 insertions(+)
 create mode 100644 t/helper/test-bitmap.c