backport.pl: tweak group names.  Specifically, don't call single-revision
groups "the rN groups"; call them just "rN".

* tools/dist/backport.pl
  (parse_entry): Set ->{header} differently.
  (merge, handle_entry, vote): Track string format change.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1509586 13f79535-47bb-0310-9956-ffa450edef68