pack-bitmap: various pack-bitmap-write cleanups

This topic was born out of tb/pseudo-merge-bitmaps, which has a few quality-of-life improvements in the pack-bitmap machinery.

The main changes are to remove the static 'struct bitmap_writer', drop one unused fields, and start using another unused one (see "move commit_positions into commit_pos fields").

There are other smaller changes, too, like cleaning up the flag allocation table in object.h, as well as introducing a new bitmap_writer_free() function.

The next round of tb/pseudo-merge-bitmaps will be based on this branch.

Thanks in advance for your review!

Taylor Blau (6):
object.h: add flags allocated by pack-bitmap.h pack-bitmap-write.c: move commit_positions into commit_pos fields
pack-bitmap: avoid use of static bitmap_writer
pack-bitmap: drop unused max_bitmaps parameter pack-bitmap-write.c: avoid uninitialized 'write_as' field
pack-bitmap: introduce bitmap_writer_free()

 builtin/pack-objects.c |  19 +++-
 midx-write.c           |  17 ++-
 object.h               |   1 +
 pack-bitmap-write.c    | 248 +++++++++++++++++++++--------------------
 pack-bitmap.h          |  38 +++++--
 5 files changed, 185 insertions(+), 138 deletions(-)

base-commit: 83f1add914c6b4682de1e944ec0d1ac043d53d78

2.45.1.151.g7cc3499008c