R1:48187cdbbe99

Fix an unusual nonterminating task graph node

Summary:
Fixes T12114. There were a couple of bugs here:

- We could draw too many joining lines if a node had a parent with multiple descendants.
- We could incorrectly ignore columns because of an `unset()`.

I //think// this fixes both things without collateral damage. This whole thing is a little hard to understand/debug and has grown beyond its original scope, so I'll probably rewrite it if there are more issues.

Test Plan:
- Unit tests.
- My…