handle unknown files in working tree passed as file_ids_from

When a working tree is passed as file_ids_from and it has unknowns it falls
over on the assert in

                if other_tree.has_filename(relative_path):
                    file_id = other_tree.path2id(relative_path)
                    assert file_id is not None
                    tt.version_file(file_id, trans_id)
                    found = True
                    break

and one a bit further down.

This should probably just be another "if" check.