Policy - lock down loadCommit() from DiffusionRequest objects
Summary: Ref T7094. The class DiffusionRequest has other public methods which use getUser() in an unguarded way. Code inspection of the call sites for loadCommit() also leads me to believe the $user is properly set.
Test Plan: clicked around diffusion a bunch and everything seemed to work okay. (happy to test any particular esoteric endpoints that come to mind)
Reviewed By: epriestley
Subscribers: Korvin, epriestley
Maniphest Tasks: T7094
Differential Revision: https://secure.phabricator.com/D11585