Previously, the case where SCMTool.get_commit_history returned None

was not graciously handled in rbt post leading to an exception. We now

explicitly handle this case and report this to the user.
Additionally, we now explicitly handle the case of a commit containing

an empty diff (e.g., if the commit was created with

git commit --allow-empty), which will not pass validation with Review

Board (as it doesn't allow empty diff uploads).

Attempted to post an empty history and the appropriate error was

  shown.
Attempted to post a history with an empty commit and the

  appropriate error was shown.