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