For a given origin, if a previous snapshot exists, this keeps the behavior of filtering
out already seen revisions from that snapshot.
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].
Improvment suggested:
- implement the "all" extid filtering after the snapshot filtering (if we can, currently i don't see how to do that)
[1] D6249