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
test-drop.t
test-inhibit.t
test-simple4server.t
tests/test-simple4server-bundle2.t