Modernize more Diffusion controllers

Summary: Ref T4245. Standardize how context is read, minor updates / modernizations / consistency tweaks.

Test Plan:

• Viewed a change.
• Viewed brnaches.
• Edited a commit.
• Viewed tags.
• Viewed history.
• Added, edited and deleted a mirror.
• Viewed push events.
• Viewed a particular event.
• Viewed ref disambiguation.
• Viewed repository list.
• Ran automation test.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4245

Differential Revision: https://secure.phabricator.com/D14944