[1.8.x] 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.

Backport of 0808ccce38 from master

tests/template_tests/test_custom.py

@@ -2,7 +2,8 @@ from __future__ import unicode_literals
 
 import os
 
-from django.template import Context, Template, TemplateSyntaxError
+from django.template import Context, Engine, Template, TemplateSyntaxError
+from django.template.base import Node
 from django.test import SimpleTestCase, ignore_warnings
 from django.test.utils import extend_sys_path
 from django.utils import six
@@ -274,6 +275,26 @@ class CustomTagTests(SimpleTestCase):
         c.use_l10n = True
         self.assertEqual(t.render(c).strip(), 'True')
 
+    def test_no_render_side_effect(self):
+        """
+        #23441 -- InclusionNode shouldn't modify its nodelist at render time.
+        """
+        engine = Engine(app_dirs=True)
+        template = engine.from_string('{% load inclusion %}{% inclusion_no_params %}')
+        count = template.nodelist.get_nodes_by_type(Node)
+        template.render(Context({}))
+        self.assertEqual(template.nodelist.get_nodes_by_type(Node), count)
+
+    def test_render_context_is_cleared(self):
+        """
+        #24555 -- InclusionNode should push and pop the render_context stack
+        when rendering. Otherwise, leftover values such as blocks from
+        extending can interfere with subsequent rendering.
+        """
+        engine = Engine(app_dirs=True)
+        template = engine.from_string('{% load inclusion %}{% inclusion_extends1 %}{% inclusion_extends2 %}')
+        self.assertEqual(template.render(Context({})).strip(), 'one\ntwo')
+
     def test_assignment_tags(self):
         c = Context({'value': 42})