docs: def entries_in_tree_oid(self, prefix, tree_oid):
          """Find the tree at this oid and return entries prefixed with ``prefix``"""
          try:
              tree = self.git.get_object(tree_oid)
          except KeyError:
              log.warning("Couldn't find object {0}".format(tree_oid))
              return empty
          else:
              return frozenset(self.entries_in_tree(prefix, tree))