HgBundle20Loader: Write nodeids to ExtID objects instead of Revision
We want ro remove Revision.metadata.
This aligns HgBundle20Loader's behavior on HgLoaderFromDisk's.
Partially resolves swh-loader-core#3139 (closed) and swh-loader-core#3138 (closed)
Migrated from D5425 (view on Phabricator)