Avoid copying task parents in TaskExecutor

As Block.copy() is potentially expensive, avoid copying the parent
structure of tasks in TaskExecutor.

lib/ansible/executor/task_executor.py

@@ -246,7 +246,8 @@ class TaskExecutor:
             task_vars[loop_var] = item
 
             try:
-                tmp_task = self._task.copy(exclude_tasks=True)
+                tmp_task = self._task.copy(exclude_parent=True, exclude_tasks=True)
+                tmp_task._parent = self._task._parent
                 tmp_play_context = self._play_context.copy()
             except AnsibleParserError as e:
                 results.append(dict(failed=True, msg=to_unicode(e)))

test/units/executor/test_task_executor.py

@@ -139,7 +139,7 @@ class TestTaskExecutor(unittest.TestCase):
 
         mock_host = MagicMock()
 
-        def _copy(exclude_block=False, exclude_tasks=False):
+        def _copy(exclude_parent=False, exclude_tasks=False):
             new_item = MagicMock()
             return new_item