Don't load the entire graph for tasks

Summary:
Ref T4788. As it turns out, our tasks are very tightly connected.

Instead of loading every parent/child task, then every parent/child of those tasks, etc., etc., only load tasks in the "same direction" that we're already heading.

For example, we load children of children, but not parents of children. And we load parents of parents, but not children of parents.

Basically we only go "up" and "down" now, but not "out" as much. This should reduce the gigantic multiple-thousand-node graphs currently shown in the UI.

I still discover the whole graph for revisiosn, because I think it's probably more useful and always much smaller. That might need adjustment too, though.

Test Plan: Seems fine locally??

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4788

Differential Revision: https://secure.phabricator.com/D16218