Find the tree at this oid and return entries prefixed with ``prefix``

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))