topic: display a hint pointing at help when a topic becomes non-empty This is an attempt at avoiding "mistakes" by new users of topics when they pull a topic and publish its changesets while being checked out. In this case, the topic will become empty but still be active; however, if this is a "foreign" topic, the user will not likely want to keep it active for the next commit operation. So we add a hint message pointing at general help of "topics" command so that they can eventually do something and learn about "hg topics --clear".