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
syntax: re
/figures/[^/]+\.png$
^html/
\.pyc$
~$
\.swp$
\.orig$
\.rej$
^tests/.*\.err$
^tests/easy_run.sh$
^build/
^dist/
^MANIFEST$
^docs/tutorials/.*\.rst$
\.ico$
tests/\.testtimes
^docs/training/graphs/
^docs/training/html/
^docs/training/index.html
^docs/training/graphviz-images/
^docs/training/img/
^docs/training/output/
^docs/training/pandocfilters/