R1:2a1b8ce85bef

For now, hard limit task graph at 100 nodes

Summary:
Ref T4788. One install has some particularly impressive task graphs which are thousands of nodes large.

The current graph is pretty broken in these cases. For now, just render a "too big to show" message. In the future, I'd plan to finesse this (e.g., show parents/children, show links to parents/children, etc).

Test Plan:
- Viewed a normal task.
- Set limit to 3, viewed a task with graph size 6, saw an error message.
- Viewed a revision stack…