backport.pl: Internal improvements.  No functional change.

* tools/dist/backport.pl
  (revert): Let the caller decide verbosity.
  (maybe_revert, handle_entry): Update callers.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1569237 13f79535-47bb-0310-9956-ffa450edef68