Improve interactive STATUS-iteration experience.

* tools/dist/backport.pl
  (main): Offer to revert at the end.  This would be useful if any 'Go ahead?'
     prompt was answered affirmatively.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1482388 13f79535-47bb-0310-9956-ffa450edef68