Create new paths in Differential
Summary: It is used by 'Pending Differential Revisions'.
Test Plan: Created a new file, arc diff, looked at this path in Diffusion, saw Pending Differential Revisions.
Reviewers: epriestley
Reviewed By: epriestley
CC: aran, Korvin
Differential Revision: https://secure.phabricator.com/D3991