Merge branch 'ps/odb-drop-whence' into ps/odb-for-each-object-filter

* 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()