Merge branch 'ps/odb-source-loose'

Merge branch 'ps/odb-source-loose'

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

* ps/odb-source-loose:
  odb/source-loose: drop pointer to the "files" source
  odb/source-loose: stub out remaining callbacks
  odb/source-loose: wire up `write_object_stream()` callback
  object-file: refactor writing objects to use loose source
  odb/source-loose: wire up `write_object()` callback
  loose: refactor object map to operate on `struct odb_source_loose`
  odb/source-loose: wire up `freshen_object()` callback
  odb/source-loose: drop `odb_source_loose_has_object()`
  odb/source-loose: wire up `count_objects()` callback
  odb/source-loose: wire up `find_abbrev_len()` callback
  odb/source-loose: wire up `for_each_object()` callback
  odb/source-loose: wire up `read_object_stream()` callback
  odb/source-loose: wire up `read_object_info()` callback
  odb/source-loose: wire up `close()` callback
  odb/source-loose: wire up `reprepare()` callback
  odb/source-loose: start converting to a proper `struct odb_source`
  odb/source-loose: store pointer to "files" instead of generic source
  odb/source-loose: move loose source into "odb/" subsystem