Re-fix the per-item diff output, (#17458)

which got lost in recent big 'performance improvements' merge by @jimi-c.

I had made a previous PR to fix this, then @bcoca had committed an
improved fix. Now it's lost again.

cf: d2b3b2c03e (lost here)
cf:  25e9b5788b (previous fix)

Earlier PR #14849
Earlier issue #14843

Please note that jimi-c broke this last time as well ... seeing a
pattern here.

lib/ansible/plugins/strategy/__init__.py

@@ -199,6 +199,9 @@ class StrategyBase:
                     elif task_result.is_skipped():
                         self._tqm.send_callback('v2_runner_item_on_skipped', task_result)
                     else:
+                        if 'diff' in task_result._result:
+                            if self._diff:
+                                self._tqm.send_callback('v2_on_file_diff', task_result)
                         self._tqm.send_callback('v2_runner_item_on_ok', task_result)
                     continue