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].
[1] D6249