Improve "thread" rendering of unusually-shaped graphs

Summary:
Ref T4788. This fixes all the bugs I was immediately able to catch:

• "Directory-Like" graph shapes could draw too many vertical lines.
• "Reverse-Directory-Like" graph shapes could draw too few vertical lines.
• Terminated, branched graph shapes drew the very last line to the wrong place.

This covers the behavior with tests, so we should be able to fix more stuff later without breaking anything.

Test Plan:

• Added failing tests and made them pass.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4788

Differential Revision: https://secure.phabricator.com/D16216