backport.pl interactive mode: Microöptimization.  Teach the state calculation
to ignore trailing whitespace when determining whether an entry has been added
to the "Don't prompt again" state.

* tools/dist/backport.pl
  (digest_entry): New.
  (parse_entry, vote): Use new function to compute entry digests.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1516082 13f79535-47bb-0310-9956-ffa450edef68