Mercurial visits are always eventful

Looking at https://grafana.softwareheritage.org/d/-lJ73Ujiz/scheduler-task-status?orgId=1&var-task=swh.loader.mercurial.tasks.LoadMercurial&var-policy=recurring&var-interval=All&from=now-30d&to=now, it looks like the mercurial loader always thinks repositories have changed since the last visit.

We should be fixing that to avoid visiting origins too often.

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Migrated from T1989 (view on Phabricator)