Preserve the cumulative path for checking includes which have parents

Otherwise, each relative include path is checked on its own, rather
than in relation to the (possibly relative) path of its parent, meaning
includes multiple level deep may fail to find the correct (or any) file.

Fixes #13472

lib/ansible/playbook/included_file.py

@@ -81,14 +81,19 @@ class IncludedFile:
                             # handle relative includes by walking up the list of parent include
                             # tasks and checking the relative result to see if it exists
                             parent_include = original_task._task_include
+                            cumulative_path = None
                             while parent_include is not None:
                                 parent_include_dir = templar.template(os.path.dirname(parent_include.args.get('_raw_params')))
+                                if cumulative_path is None:
+                                    cumulative_path = parent_include_dir
+                                elif not os.path.isabs(cumulative_path):
+                                    cumulative_path = os.path.join(parent_include_dir, cumulative_path)
                                 include_target = templar.template(include_result['include'])
                                 if original_task._role:
-                                    new_basedir = os.path.join(original_task._role._role_path, 'tasks', parent_include_dir)
+                                    new_basedir = os.path.join(original_task._role._role_path, 'tasks', cumulative_path)
                                     include_file = loader.path_dwim_relative(new_basedir, 'tasks', include_target)
                                 else:
-                                    include_file = loader.path_dwim_relative(loader.get_basedir(), parent_include_dir, include_target)
+                                    include_file = loader.path_dwim_relative(loader.get_basedir(), cumulative_path, include_target)
 
                                 if os.path.exists(include_file):
                                     break