Extend the git-cl-comments command functionality

The "comments" command is taught to:
- accept an arbitrary review issue id;
- post comments on the current or specified review issue.

This is required for the auto-rebaseline bot to notify a
rebaseline requestor of the fact that the rebaseline has happened
by means of posting a comment to the review issue that requested
the rebaseline, as suggested by iannucci@.

R=iannucci, szager, dpranke

Review URL: https://codereview.chromium.org/693873002

git-svn-id: http://src.chromium.org/chrome/trunk/tools/depot_tools@292844 4ff67af0-8c30-449e-8e8b-ad334ec8d88c