backfill: Add missing raw_manifest to directories
This was not covered by tests so far, because swh.model.tests.swh_model_data.TEST_OBJECTS did not contain any object with a raw_manifest.
But it will in swh-model > 5.0.0 (thanks to swh-model!223 (closed))
Migrated from D7359 (view on Phabricator)