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
105a22cf69