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