backport.pl: Relax parsing.

* tools/dist/backport.pl
  (parse_entry): Ignore trailing whitespace.  This matters, for example, for
    entry headers such as " * 1.8.x-r42 branch ".

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1567557 13f79535-47bb-0310-9956-ffa450edef68