Merge branch 'tb/cruft-packs'

Merge branch 'tb/cruft-packs'

A mechanism to pack unreachable objects into a "cruft pack",
instead of ejecting them into loose form to be reclaimed later, has
been introduced.

* tb/cruft-packs:
  sha1-file.c: don't freshen cruft packs
  builtin/gc.c: conditionally avoid pruning objects via loose
  builtin/repack.c: add cruft packs to MIDX during geometric repack
  builtin/repack.c: use named flags for existing_packs
  builtin/repack.c: allow configuring cruft pack generation
  builtin/repack.c: support generating a cruft pack
  builtin/pack-objects.c: --cruft with expiration
  reachable: report precise timestamps from objects in cruft packs
  reachable: add options to add_unseen_recent_objects_to_traversal
  builtin/pack-objects.c: --cruft without expiration
  builtin/pack-objects.c: return from create_object_entry()
  t/helper: add 'pack-mtimes' test-tool
  pack-mtimes: support writing pack .mtimes files
  chunk-format.h: extract oid_version()
  pack-write: pass 'struct packing_data' to 'stage_tmp_packfiles'
  pack-mtimes: support reading .mtimes files
  Documentation/technical: add cruft-packs.txt