backport.pl: use automatic merge when $SVN ≥ 1.8.0.
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: f8a4e5e023278da1e04e203c7fe051e3c4285d88@1468166 13f79535-47bb-0310-9956-ffa450edef68

YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion