Use loop_control.loop_var directly

6eefc11c converted task.loop_control into an object, but while the other
callers were updated to use .loop_var instead of .get('loop_var'), this
site was overlooked.

This can be reproduced by including with loop_control a file that does
set_fact; a simple regression test along these lines is included.

(cherry picked from commit 950cc26aab)

lib/ansible/executor/process/result.py

@@ -173,7 +173,7 @@ class ResultProcess(multiprocessing.Process):
                             # if this task is registering facts, do that now
                             loop_var = 'item'
                             if result._task.loop_control:
-                                loop_var = result._task.loop_control.get('loop_var') or 'item'
+                                loop_var = result._task.loop_control.loop_var or 'item'
                             item = result_item.get(loop_var, None)
                             if result._task.action == 'include_vars':
                                 for (key, value) in iteritems(result_item['ansible_facts']):

test/integration/test_includes.yml

@@ -1 +1,3 @@
 - include: test_includes2.yml parameter1=asdf parameter2=jkl
+
+- include: test_includes3.yml

test/integration/test_includes3.yml

@@ -0,0 +1,7 @@
+---
+- hosts: localhost
+  tasks:
+    - include: test_includes4.yml
+      with_items: ["a"]
+      loop_control:
+        loop_var: r

test/integration/test_includes4.yml

@@ -0,0 +1,2 @@
+- set_fact:
+    p: 1