Sync 'ds/multi-pack-index' to v2.19.0-rc0

Sync 'ds/multi-pack-index' to v2.19.0-rc0

* ds/multi-pack-index: (23 commits)
  midx: clear midx on repack
  packfile: skip loading index if in multi-pack-index
  midx: prevent duplicate packfile loads
  midx: use midx in approximate_object_count
  midx: use existing midx when writing new one
  midx: use midx in abbreviation calculations
  midx: read objects from multi-pack-index
  config: create core.multiPackIndex setting
  midx: write object offsets
  midx: write object id fanout chunk
  midx: write object ids in a chunk
  midx: sort and deduplicate objects from packfiles
  midx: read pack names into array
  multi-pack-index: write pack names in chunk
  multi-pack-index: read packfile list
  packfile: generalize pack directory list
  t5319: expand test data
  multi-pack-index: load into memory
  midx: write header information to lockfile
  multi-pack-index: add 'write' verb
  ...