Merge branch 'tb/incremental-midx-part-3.2'

Merge branch 'tb/incremental-midx-part-3.2'

Further work on incremental repacking using MIDX/bitmap

* tb/incremental-midx-part-3.2:
  midx: enable reachability bitmaps during MIDX compaction
  midx: implement MIDX compaction
  t/helper/test-read-midx.c: plug memory leak when selecting layer
  midx-write.c: factor fanout layering from `compute_sorted_entries()`
  midx-write.c: enumerate `pack_int_id` values directly
  midx-write.c: extract `fill_pack_from_midx()`
  midx-write.c: introduce `midx_pack_perm()` helper
  midx: do not require packs to be sorted in lexicographic order
  midx-write.c: introduce `struct write_midx_opts`
  midx-write.c: don't use `pack_perm` when assigning `bitmap_pos`
  t/t5319-multi-pack-index.sh: fix copy-and-paste error in t5319.39
  git-multi-pack-index(1): align SYNOPSIS with 'git multi-pack-index -h'
  git-multi-pack-index(1): remove non-existent incompatibility
  builtin/multi-pack-index.c: make '--progress' a common option
  midx: introduce `midx_get_checksum_hex()`
  midx: rename `get_midx_checksum()` to `midx_get_checksum_hash()`
  midx: mark `get_midx_checksum()` arguments as const