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