The pack-bitmap machinery learned to write pseudo-merge bitmaps,
which act as imaginary octopus merges covering un-bitmapped
reference tips. This enhances bitmap coverage, and thus,
performance, for repositories with many references using bitmaps.

* tb/pseudo-merge-reachability-bitmap: (24 commits)
  t/perf: implement performace tests for pseudo-merge bitmaps
  pseudo-merge: implement support for finding existing merges
  ewah: `bitmap_equals_ewah()`
  pack-bitmap: extra trace2 information
  pack-bitmap.c: use pseudo-merges during traversal
  t/test-lib-functions.sh: support `--date` in `test_commit_bulk()`
  pack-bitmap: implement test helpers for pseudo-merge
  ewah: implement `ewah_bitmap_popcount()`
  pseudo-merge: implement support for reading pseudo-merge commits
  pack-bitmap.c: read pseudo-merge extension
  pseudo-merge: scaffolding for reads
  pack-bitmap: extract `read_bitmap()` function
  pack-bitmap-write.c: write pseudo-merge table
  pack-bitmap-write.c: select pseudo-merge commits
  pseudo-merge: implement support for selecting pseudo-merge commits
  pack-bitmap: make `bitmap_writer_push_bitmapped_commit()` public
  pack-bitmap: implement `bitmap_writer_has_bitmapped_object_id()`
  pack-bitmap-write: support storing pseudo-merge commits
  pseudo-merge.ch: initial commit
  pack-bitmap: move some initialization to `bitmap_writer_init()`
  ...