Fixed #23441, #24555 -- Improved the behavior of InclusionNode.

This change:

* Makes the InclusionNode cache-safe by removing render-time side effects
  to its nodelist.
* Ensures the render_context stack is properly scoped and reset by updating
  the render call to use Template.render rather than Nodelist.render.

tests/template_tests/templates/inclusion_base.html

@@ -0,0 +1 @@
+{% block content %}base{% endblock %}