backport.pl: Sanitify backup-diff generation.  No functional change.
backport.pl: Sanitify backup-diff generation.  No functional change.

* tools/dist/backport.pl
  (merge): Concentrate all the logic governing the backup diff in one place.

git-svn-id: f8a4e5e023278da1e04e203c7fe051e3c4285d88@1569239 13f79535-47bb-0310-9956-ffa450edef68

YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion