topic: suggest to clear a topic that becomes empty
We add a hint suggesting "hg topic --clear" when a topic becomes empty.
This usually happens when all its changesets get public, that is when
the user does a 'phase -p' or when they pull from a server. To
discriminate relevant cases for which we should display the hint (and
typically exclude explicit 'hg topic --clear' operation), we look for
the transaction "type" to only consider 'phase' or 'push' transaction.
In the latter case, the transaction name is not simply 'push-response'
but it also included the remote URL so we need to match on string
content.