Follow-up to r1471243 (on 1.8.x):

* tools/dist/backport.pl
  (merge): Croak if the merge caused no-non-mergeinfo changes.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1471467 13f79535-47bb-0310-9956-ffa450edef68