Ref T8126. See that task for discussion. This change:

• Updates language to be more consistent ("Parents", "Subtasks") since I moved us away from the often-confusing "Block" language in T4788.
• Fixes bugs with finding the wrong set of tasks if tasks have a mixture of open and closed subtasks or parents.

• Created four tasks: no subtasks, one closed subtask, one open subtask, mixture of open and closed subtasks.
• Created four more tasks: no parents, one closed parent, one open parent, mixture of open and closed parents.
• Searched for all this stuff, got the proper results: