R1:55f4a258d2de

When updating revisions in responset to commits, use the omnipotent viewer to pull diffs

Summary:
Ref T13625. See that task for discussion.

Currently, the Viewer when performing revision updates in response to commits may be an arbitrary low-privilege user (an Application, a disabled User, a bot, a mailing list, etc).

Today, this leads to an exception when trying to make API calls.

Ideally, we probably would not perform the update in these cases. However, performing the update isn't a policy violation…