When updating to a bookmark, mention that the bookmark is now
active. This is a reminder that update does not move the
current bookmark if an explicit target is given - instead
it activates that target.
Previously the fncache was cleaned up at read time by noticing when it was out
of sync. This caused writes to happen outside the scope of transactions and
could have caused race conditions. With this change, we'll keep the fncache
up-to-date as we go by removing old entries during repair.strip.
