Merge branch 'ps/odb-drop-whence' into jch

Merge branch 'ps/odb-drop-whence' into jch

The whence field in struct object_info has been removed,
refactoring backend-specific object information retrieval into an
opt-in struct object_info_source structure.

* ps/odb-drop-whence:
  odb: document object info fields
  odb: drop `whence` field from object info
  treewide: convert users of `whence` to the new source field
  odb: add `source` field to struct object_info_source
  odb: make backend-specific fields optional
  packfile: thread odb_source_packed through packed_object_info()