backport.pl: use automatic merge when $SVN ≥ 1.8.0.

* tools/dist/backport.pl
  ($SVNvsn): New global.
  (merge): Omit --reintegrate for 1.8.  Adjust messages accordingly.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1468166 13f79535-47bb-0310-9956-ffa450edef68