Fixed #13373 -- Ensured that {% if %} statements will short circuit template...

Fixed #13373 -- Ensured that {% if %} statements will short circuit template logic and not evaluate clauses that don't require evaluation. Thanks to Jerry Stratton for the report.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@13001 bcc190cf-cafb-0310-a4f2-bffc1f526a37