Improve --fail-on-template-vars: use origin if available

This will be there with Django 1.9+ always, and in case of
`settings.TEMPLATE_DEBUG` before.
It stops going up to the Template, but uses the nearest location (which
is required when extending templates).

Using `django.template.base.Origin` also gives the benefit of having the
full/absolute path.

pytest_django/plugin.py

@@ -488,10 +488,24 @@ def __contains__(self, key):
             """There is a test for '%s' in TEMPLATE_STRING_IF_INVALID."""
             return key == '%s'
 
-        def _get_template(self):
+        def _get_origin(self):
+            stack = inspect.stack()
+
+            # Try to use topmost `self.origin` first (Django 1.9+, and with
+            # TEMPLATE_DEBUG)..
+            for f in stack[2:]:
+                func = f[3]
+                if func == 'render':
+                    frame = f[0]
+                    try:
+                        origin = frame.f_locals['self'].origin
+                    except (AttributeError, KeyError):
+                        continue
+                    if origin is not None:
+                        return origin
+
             from django.template import Template
 
-            stack = inspect.stack()
             # finding the ``render`` needle in the stack
             frame = reduce(
                 lambda x, y: y[3] == 'render' and 'base.py' in y[1] and y or x,
@@ -507,14 +521,14 @@ def _get_template(self):
             # ``django.template.base.Template``
             template = f_locals['self']
             if isinstance(template, Template):
-                return template
+                return template.name
 
         def __mod__(self, var):
             """Handle TEMPLATE_STRING_IF_INVALID % var."""
-            template = self._get_template()
-            if template:
+            origin = self._get_origin()
+            if origin:
                 msg = "Undefined template variable '%s' in '%s'" % (
-                    var, template.name)
+                    var, origin)
             else:
                 msg = "Undefined template variable '%s'" % var
             if self.fail:

tests/test_environment.py

@@ -73,6 +73,10 @@ def invalid_template(request):
         """, 'views.py')
     django_testdir.create_app_file(
         "<div>{{ invalid_var }}</div>",
+        'templates/invalid_template_base.html'
+    )
+    django_testdir.create_app_file(
+        "{% extends 'invalid_template_base.html' %}",
         'templates/invalid_template.html'
     )
     django_testdir.create_test_module('''
@@ -86,9 +90,14 @@ def test_ignore(client):
             client.get('/invalid_template/')
         ''')
     result = django_testdir.runpytest_subprocess('-s', '--fail-on-template-vars')
+
+    if get_django_version() >= (1, 9):
+        origin = "'*/tpkg/app/templates/invalid_template_base.html'"
+    else:
+        origin = "'invalid_template.html'"
     result.stdout.fnmatch_lines_random([
         "tpkg/test_the_test.py F.",
-        "Undefined template variable 'invalid_var' in 'invalid_template.html'",
+        "Undefined template variable 'invalid_var' in {}".format(origin)
     ])