Fixed #7318 -- Cleaned up the template inheritance logic, specifically to handle the case where the parent template has no template tags/blocks. Took the opportunity to optimize the logic a little. Thanks to Matthias Kestenholz <mk@spinlock.ch> for the original report and test case.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@7688 bcc190cf-cafb-0310-a4f2-bffc1f526a37