Use extids to filter out already seen revisions across hg origins
For a given origin, if a previous snapshot exists, this keeps the behavior of filtering out already seen revisions from that snapshot. Then it goes on and tries to filter the remaining revisions through the extid mappings for other mercurial origins.
Note: This makes me think back that we need to update the storage interface so the filtering on the extid_version can happen server side. So as to avoid filtering in the client code. This is somewhat an independent work from this which got started in a dedicated diff [1].
Test Plan
tox
Migrated from D6240 (view on Phabricator)