tb/cruft-packs (was Re: What's cooking in git.git (Mar 2022, #01; Thu, 3))

> * tb/cruft-packs (2022-03-02) 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
> 
>  A mechanism to pack unreachable objects into a "cruft pack",
>  instead of ejecting them into loose form to be reclaimed later, has
>  been introduced.
> 
>  Will merge to 'next'?
>  source: <cover.1646266835.git.me@ttaylorr.com>