R1:646280972b88

Glue the new FormationView on top of the older Filetree view in Differential

Summary: Ref T13516. This glues "FormationView" to "ChangesetList". The actual tree is not functional in any meaningful way yet.

Test Plan: {F7373838}

Maniphest Tasks: T13516

Differential Revision: https://secure.phabricator.com/D21151