Merge branch 'tb/repack-geometric-cruft' into seen

Merge branch 'tb/repack-geometric-cruft' into seen

'git repack' has been taught to accept '--geometric' and '--cruft'
together.  When both are given, non-cruft packs are rolled up by the
geometric repack as usual, while a separate cruft pack is written to
collect unreachable objects.

* tb/repack-geometric-cruft:
  SQUASH??? bare grep !???
  repack: support combining '--geometric' with '--cruft'
  pack-objects: support '--refs-snapshot' with 'follow-reachable'
  pack-objects: introduce '--stdin-packs=follow-reachable'
  pack-objects: extract `stdin_packs_add_all_pack_entries()`
  repack-geometry: drop unused redundant-pack removal
  repack: delete geometric packs via existing_packs
  repack: teach MIDX retention about geometric rollups
  repack: mark geometric progression of packs as retained
  repack: extract `locate_existing_pack()` helper
  repack: unconditionally exclude non-kept packs