mercurial.from_disk: Filter out globally already seen hg_nodeid

This exploits the extid mappings for all seen origins and does not limit the filtering to only happen when we visit an already seen origin.

In another commit, this also drops an unnecessary flush instruction.

Test Plan

tox (failing as now plenty of uneventful events happen instead of eventful, probably either some missing reset or some scenario change needs to happen)

---

Migrated from D6188 (view on Phabricator)