backport.pl: Forcefully disable "run merges and commit them" mode.

* tools/dist/backport.pl
  ($MAY_COMMIT / svn-role mode): Bail out if this mode was requested.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1589871 13f79535-47bb-0310-9956-ffa450edef68