backport.pl: Follow-up to r1507640: implement basename() really correctly
(that is, don't).

* tools/dist/backport.pl
  (File::Basename::basename): Import.
  (usage, handle_entry): Use it instead of reinventing it.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1507643 13f79535-47bb-0310-9956-ffa450edef68