Enable 'jumping to toc' on diffusion commit page

Summary:
Diffusion page is sharing the keyboard shortcuts code with
Differential page. But since the toc (Changes) panel doesn't have id
'differential-review-toc', the 'jumping to toc' doesn't work. The fix is
to add the ID. I don't like adding 'Differential' to the Diffusion page.
Later we should refactor the code to extract the shared components out of Differential.

Test Plan:
verified that 't' worked on the diffusion commit page.

Reviewers: epriestley, nh

Reviewed By: epriestley

CC: hwang, aran, Korvin

Differential Revision: https://secure.phabricator.com/D2500