Merge branch 'tb/pack-bitmap-write-cleanups'

Merge branch 'tb/pack-bitmap-write-cleanups'

The pack bitmap code saw some clean-up to prepare for a follow-up topic.

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