backport.pl: Housekeeping.

Make all the global boolean knobs 0/1-valued, and introduce
a wrapper for use in merge()'s shell script.

* tools/dist/backport.pl
  (@sh): New.
  ($MAY_COMMIT, $DEBUG): Convert to Perl-boolean from "$shellscript"-boolean.
  ($YES): Ditto, and note merge() would DTWT if "YES=0" was in the environment.
  (merge): Track changes.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1568596 13f79535-47bb-0310-9956-ffa450edef68