Switch back to janky array copying

Summary: Bad news @cpojer @tomo. IE8 doesn't like you.

Test Plan: Load a diff in IE8; see changes and don't get JS errors.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, tomo, Korvin, cpojer

Differential Revision: https://secure.phabricator.com/D8728