tb/cruft-packs (was: Re: What's cooking in git.git (Jan 2022, #02; Wed, 5))

> * tb/cruft-packs (2021-11-29) 17 commits
>  . 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
>
>  Instead of leaving unreachable objects in loose form when packing,
>  or ejecting them into loose form when repacking, gather them in a
>  packfile with an auxiliary file that records the last-use time of
>  these objects.
>
>  Expecting a reroll.
>  cf. <865b99dd-0b18-9a07-49c1-3959a777c685@gmail.com>
>  cf. <c9437c89-9258-4034-9886-8a2aec46aa6b@gmail.com>
>  cf. <YaqiYGM48p5F9lS1@nand.local>
>  source: <cover.1638224692.git.me@ttaylorr.com>