backport.pl: Clean up revert-before-merge logic.  No functional change.

* tools/dist/backport.pl
  (merge): Concentrate all the logic governing revert in one place.
  (revert): Grow optional discard_STATUS argument.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1569242 13f79535-47bb-0310-9956-ffa450edef68