backport.pl: Add an assertion.
backport.pl: Add an assertion.

* tools/dist/backport.pl
  (prompt): Assert that $YES is unset.

git-svn-id: f8a4e5e023278da1e04e203c7fe051e3c4285d88@1482391 13f79535-47bb-0310-9956-ffa450edef68

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