Fix undefinded $changesets when submitting inline comments in Differential

Summary: Fixes T5262. This branch is overzealous, and causes us to fail to load changeses if `metamta.differential.unified-comment-context` is off. It was on for me locally for testing, which is why I missed this.

Test Plan: No more exception.

Reviewers: chad

Reviewed By: chad

Subscribers: richardvanvelzen, epriestley

Maniphest Tasks: T5262

Differential Revision: https://secure.phabricator.com/D9376