Merge branch 'tb/pseudo-merge-bitmap-fixes'

Merge branch 'tb/pseudo-merge-bitmap-fixes'

We created a useless pseudo-merge reachability bitmap that is about
0 commits, and attempted to include commits that are not in packs,
which made no sense.  These bugs have been corrected.

* tb/pseudo-merge-bitmap-fixes:
  pseudo-merge.c: ensure pseudo-merge groups are closed
  pseudo-merge.c: do not generate empty pseudo-merge commits
  t/t5333-pseudo-merge-bitmaps.sh: demonstrate empty pseudo-merge groups
  pack-bitmap-write.c: select pseudo-merges even for small bitmaps
  pack-bitmap: drop redundant args from `bitmap_writer_finish()`
  pack-bitmap: drop redundant args from `bitmap_writer_build()`
  pack-bitmap: drop redundant args from `bitmap_writer_build_type_index()`
  pack-bitmap: initialize `bitmap_writer_init()` with packing_data