When a directory has a single directory child, collapse them into a single "a/b/" path entry

Summary:
Ref T13516. Instead of rendering trees like this:

• a/
  • b/
    • c.txt

...render:

• a/b/
  • c.txt

Test Plan: {F7374205}

Maniphest Tasks: T13516

Differential Revision: https://secure.phabricator.com/D21155