nominate.pl interactive mode: grow "show only entries matching a pattern" mode.

* tools/dist/backport.pl
  (backport_usage): Document new feature.
  (handle_entry): Grow an optional pattern argument, use it to filter entries.
  (backport_main): Parse pattern from argv and pass it to handle_entry().
  (): Forward @ARGV to backport_main().


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1515095 13f79535-47bb-0310-9956-ffa450edef68