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 local repro is clean now:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12114

Differential Revision: https://secure.phabricator.com/D17211