Render the state tree in "arc branches" slightly more cleanly

Summary:
Ref T13546. Try using unicode box drawing characters to render a more obvious tree struture in "arc branches".

Unclear if this has enough support to use, but seems okay so far.

Test Plan: Ran "arc branches", saw a nicer tree display.

Maniphest Tasks: T13546

Differential Revision: https://secure.phabricator.com/D21390