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