Merge branch 'tb/repack-existing-packs-cleanup'

Merge branch 'tb/repack-existing-packs-cleanup'

The code to keep track of existing packs in the repository while
repacking has been refactored.

* tb/repack-existing-packs-cleanup:
  builtin/repack.c: extract common cruft pack loop
  builtin/repack.c: avoid directly inspecting "util"
  builtin/repack.c: store existing cruft packs separately
  builtin/repack.c: extract `has_existing_non_kept_packs()`
  builtin/repack.c: extract redundant pack cleanup for existing packs
  builtin/repack.c: extract redundant pack cleanup for --geometric
  builtin/repack.c: extract marking packs for deletion
  builtin/repack.c: extract structure to store existing packs