code: func (a *Driver) DiffGetter(id string) (graphdriver.FileGetCloser, error) {
  	p := path.Join(a.rootPath(), "diff", id)
  	return fileGetNilCloser{storage.NewPathFileGetter(p)}, nil
  }
docs: // DiffGetter returns a FileGetCloser that can read files from the directory that
  // contains files for the layer differences. Used for direct access for tar-split.