R1:874ae4b7ee57

Allow anonymous access to conduit getdiff method.

Summary:
Allow anonymous access to conduit getdiff method, which is needed for anonymous
`arc patch`.

Test Plan: Running getdiff with an unauthenticated conduit should work.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: 1068