GitLab

This subcommand prints the trybot configuration in the
buildbucket.config format expected by gerrit. This should hopefully
make it a little easier to maintain the buildbucket.config file.

Bug: 
Change-Id: I7d956efb6c08128ae0ceaa6b68bcc9001296549c
Reviewed-on: https://chromium-review.googlesource.com/568666Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Peter Collingbourne <pcc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491647}