Probably fix bad method call in Diffusion

Summary: Fixes T5869. Ref T4896. This `setID()` method no longer exists.

Test Plan: (WARNING) This is a pain to reproduce locally so I'm just winging it. I'm 99% sure this ID is only used to generate an anchor link. This is a hack to start with, and T4896 will eventualy clean it up properly.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4896, T5869

Differential Revision: https://secure.phabricator.com/D10254