Merge branch 'ps/packfile-store' into tb/incremental-midx-part-3.1

Merge branch 'ps/packfile-store' into tb/incremental-midx-part-3.1

* ps/packfile-store:
  packfile: refactor `get_packed_git_mru()` to work on packfile store
  packfile: refactor `get_all_packs()` to work on packfile store
  packfile: refactor `get_packed_git()` to work on packfile store
  packfile: move `get_multi_pack_index()` into "midx.c"
  packfile: introduce function to load and add packfiles
  packfile: refactor `install_packed_git()` to work on packfile store
  packfile: split up responsibilities of `reprepare_packed_git()`
  packfile: refactor `prepare_packed_git()` to work on packfile store
  packfile: reorder functions to avoid function declaration
  odb: move kept cache into `struct packfile_store`
  odb: move MRU list of packfiles into `struct packfile_store`
  odb: move packfile map into `struct packfile_store`
  odb: move initialization bit into `struct packfile_store`
  odb: move list of packfiles into `struct packfile_store`
  packfile: introduce a new `struct packfile_store`