topic: improve UX by using pager only when we should
Before this patch we were using pager in every case of topic command,
either it is setting up a new topic or clearing the active topic.
After this patch it will not use pager in following commands:
$ hg topic [TOPIC] -r [rev]
$ hg topic --clear
evolvecmd: _possibledestination() skip visiting empty parent
I see that even in the case when "parent == -1" we are visiting
that also and find succsets for that and in last -1 is being
added in the set of possible destination. Why not skip that?