Print "git cl comments" json to stdout when the output file is "-"

Change-Id: I91021317e52019a0d499cccc4f40f062127b300e
Reviewed-on: https://chromium-review.googlesource.com/1228094
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>