R1:15c7eb142570

When a task graph has too much stuff, only show adjacent nodes (direct parents/children)

Summary:
Ref T4788. This gives us a new level of graceful degradation, so now we show:

- Zero through 100 connected tasks: whole graph.
- More than 100 connnected tasks, but fewer than 100 direct parents/subtasks: just parents and subtasks, with "..." to hint that the graph is cut off.
- More than 100 parents and children: just the "sorry, too much stuff" error message.

Test Plan: {F1740882}

Reviewers: chad…