Replace "PhutilFileTree" with a more abstract "VectorTree"
Ref T13520. Replace "FileTree" with a "VectorTree" that does roughly the same thing. The major goals are:
- Compress trees which contain sequences of child directories with no sibilings.
- Build hierarchies of paths where path components may include renames.
This is approximately similar to "FileTree" and similar to client logic in the new paths panel.
Test Plan: See next change.
Maniphest Tasks: T13520
Differential Revision: https://secure.phabricator.com/D21182