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()

diff --cc builtin/cat-file.c
Simple merge

diff --cc builtin/pack-objects.c
Simple merge

diff --cc odb/source-inmemory.c
Simple merge

diff --cc odb/source-loose.c
Simple merge

diff --cc odb/source-packed.c
Simple merge