backport.pl interactive mode: eliminate needless "Revert?" prompts.

To reproduce:
    echo ynq | backport.pl

* tools/dist/backport.pl
  (revert): Reset the global counter $MERGED_SOMETHING so that we don't call
    this function again in the exit path.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1508700 13f79535-47bb-0310-9956-ffa450edef68