Don't assign both parent blocks and task includes to blocks

This causes problems when fetching parent attributes, as the include
was being skipped because the parent block would fetch the attribute
from the parent play first.

Fixes #13872

lib/ansible/playbook/block.py

@@ -44,12 +44,17 @@ class Block(Base, Become, Conditional, Taggable):
     def __init__(self, play=None, parent_block=None, role=None, task_include=None, use_handlers=False, implicit=False):
         self._play         = play
         self._role         = role
-        self._task_include = task_include
-        self._parent_block = parent_block
+        self._task_include = None
+        self._parent_block = None
         self._use_handlers = use_handlers
         self._implicit     = implicit
         self._dep_chain    = []
 
+        if task_include:
+            self._task_include = task_include
+        elif parent_block:
+            self._parent_block = parent_block
+
         super(Block, self).__init__()
 
     def get_vars(self):