HgLoaderFromDisk: Read and write ExtIDs to find revisions already loaded
For now, ExtIDs are used in addition to revision metadata. But in the near future, we want to migrate nodeids from revision metadata to the ExtID storage, and drop all revision metadata.
Depends on !55 (closed) and !56 (closed)
Resolves swh-loader-core#3140 (closed) and swh-loader-core#3142 (closed) for this loader
Test Plan
Tests fail because they depend on swh-storage!662 (closed)
Migrated from D5371 (view on Phabricator)