Fix issues guessing review requests in rbt land.
Review Request #6726 — Created Jan. 2, 2015 and submitted
rbt land wasn't including the specified branch name as part of the list of arguments that get passed to get_revisions(). This caused the review request guessing code to look at HEAD, instead of the specified branch. That bug exposed another problem, which is that the review request guessing code didn't pre-define the summary and description, leading to an error later on if a summary or description was not found. We now properly bail in this case.
Properly landed a change from a different branch than HEAD.