def fetch_objects(self, determine_wants, graph_walker, progress,
                      get_tagged=None):
        raise NotImplementedError
build() method can write to a custom file object.
dataset/ETHPy150Open natestedman/Observatory/observatory/lib/dulwich/server.py/BackendRepo.fetch_objects--crossover with--dataset/ETHPy150Open miracle2k/webassets/tests/test_bundle_build.py/TestBuildVarious.test_with_custom_output