backport.pl: Fix a bug with current 1.7.x/STATUS whereby it would hang trying
to read from stdin.

* tools/dist/backport.pl
  (main): Do away with ARGV magic.

Found by: breser

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1462316 13f79535-47bb-0310-9956-ffa450edef68