Future proof activity log parsing

When the log has an activity that is not known, eg added by a future
version of git-annex, it used to be treated as no activity at all,
which would make git-annex expire think it should expire the repository,
despite it having some kind of recent activity.

Hopefully there will be no reason to add a new activity until enough
time has passed that this commit is in use everywhere.

Sponsored-by: Jake Vosloo on Patreon