Jump to new mentions even if we're already in mentions.

Jump to new mentions even if we're already in mentions.

I noticed this because of the bug fixed in the previous commit - I
_wasn't_ already in my mentions feed, and got thrown into it by an
incoming message, but the file position didn't update.

If the ReadMentions activity state had been properly GCed and
recreated, it would have jumped automatically to the new message. But
it _ought_ to be doing that even if I were in Read Mentions already,
which is a rarer occurrence. So the previous bug gave me the
opportunity to spot and fix this one too, before it hit for real!