Reverting an activity should either remove the last state or immediately save

I tend to use `revert` on an activity when it has "drifted" and the buffers shown have changed. The new annotations PR #83 revealed that the `last` state remains set even after `revert`.

This is surprising because that state is no longer reachable while the activity is active, AFAIU, and it will eventually be overwritten when save-all is run during idle time. So it seems sensible either to remove the last state, or immediately save the activity, after it gets reverted.