Merge branch 'ps/packed-git-in-object-store' into next

Merge branch 'ps/packed-git-in-object-store' into next

The list of packfiles used in a running Git process is moved from
the packed_git structure into the packfile store.

* ps/packed-git-in-object-store:
  packfile: track packs via the MRU list exclusively
  packfile: always add packfiles to MRU when adding a pack
  packfile: move list of packs into the packfile store
  builtin/pack-objects: simplify logic to find kept or nonlocal objects
  packfile: fix approximation of object counts
  http: refactor subsystem to use `packfile_list`s
  packfile: move the MRU list into the packfile store
  packfile: use a `strmap` to store packs by name