Fix changeset loading issue for "Download Raw Diff" in Differential

Summary: Fixes T5309. Modernize this callsite to use ChangesetQuery and pick up attached objects.

Test Plan: Clicked "Download Raw Diff" in Differential.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T5309

Differential Revision: https://secure.phabricator.com/D9461