Merge branch 'tb/repack-write-midx' into tb/fix-midx-rename-while-mapped

Merge branch 'tb/repack-write-midx' into tb/fix-midx-rename-while-mapped

* tb/repack-write-midx:
  test-read-midx: fix leak of bitmap_index struct
  builtin/repack.c: pass `--refs-snapshot` when writing bitmaps
  builtin/repack.c: make largest pack preferred
  builtin/repack.c: support writing a MIDX while repacking
  builtin/repack.c: extract showing progress to a variable
  builtin/repack.c: rename variables that deal with non-kept packs
  builtin/repack.c: keep track of existing packs unconditionally
  midx: preliminary support for `--refs-snapshot`
  builtin/multi-pack-index.c: support `--stdin-packs` mode
  midx: expose `write_midx_file_only()` publicly