Fix backport script to not require Branch: header.

This makes it so that if the revision line has the branch on it and there is
no Branch: header the merge will complete rather than failing.

* tools/dist/backport.pl
  (merge): Make the branch header prefix optional when searching for the entry
    to remove before committing.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1569068 13f79535-47bb-0310-9956-ffa450edef68