Commit improvements

This patch series is in relation to the discussion that were made on the mailing list. 

Brief Summary:
* I removed the "update--commit--message" item from the table, since it's unclear what this item does. So instead, when amending, we just auto fill the commit window with the last commit message.
* Added generic API so that the output window no longer shows when canceling the commit. 

The approach for not showing the output window, when canceling the commit, is quite different than what I previous shared (https://github.com/textmate/git.tmbundle/pull/24). I think this is better since it would easily allow for similar changes in the future.

Let me know if you have any comments or concerns.