Merge branch 'ps/odb-source-packed' into jch

Merge branch 'ps/odb-source-packed' into jch

The packed object source has been refactored into a proper struct
odb_source.

* ps/odb-source-packed:
  odb/source-packed: drop pointer to "files" parent source
  midx: refactor interfaces to work on "packed" source
  odb/source-packed: stub out remaining functions
  odb/source-packed: wire up `freshen_object()` callback
  odb/source-packed: wire up `find_abbrev_len()` callback
  odb/source-packed: wire up `count_objects()` callback
  odb/source-packed: wire up `for_each_object()` callback
  odb/source-packed: wire up `read_object_stream()` callback
  odb/source-packed: wire up `read_object_info()` callback
  packfile: use higher-level interface to implement `has_object_pack()`
  odb/source-packed: wire up `reprepare()` callback
  odb/source-packed: wire up `close()` callback
  odb/source-packed: start converting to a proper `struct odb_source`
  odb/source-packed: store pointer to "files" instead of generic source
  packfile: move packed source into "odb/" subsystem
  packfile: split out packfile list logic
  packfile: rename `struct packfile_store` to `odb_source_packed`