nominate.pl: optionally revert STATUS when done.

* tools/dist/backport.pl
  (check_local_mods_to_STATUS): New sub, factored out from..
  (backport_main): .. here.
  (nominate_main): Check for local mods. 
    Offer to revert STATUS if not committing.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1511636 13f79535-47bb-0310-9956-ffa450edef68