Fixed a bug in jinja2/debug (second time, forgot to merge and threw away changes -.-)

--HG--
branch : trunk
diff --git a/jinja2/debug.py b/jinja2/debug.py
index b6a64e3..aa78ef0 100644
--- a/jinja2/debug.py
+++ b/jinja2/debug.py
@@ -184,7 +184,12 @@
     globals = {
         '__name__':             filename,
         '__file__':             filename,
-        '__jinja_exception__':  exc_info[:2]
+        '__jinja_exception__':  exc_info[:2],
+
+        # we don't want to keep the reference to the template around
+        # to not cause circular dependencies, but we mark it as Jinja
+        # frame for the ProcessedTraceback
+        '__jinja_template__':   None
     }
 
     # and fake the exception