Fix backport.pl so it doesn't ignore the Branch if the value ends in /.

* tools/dist/backport.pl
  (sanitize_branch): Be less greedy with removing leading characters by using
    the $BRANCHES constant to find the start of the branch.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1541813 13f79535-47bb-0310-9956-ffa450edef68