Don't display old lines on right side in diff

Summary: I've done the same stupid copy/paste mistake twice in one day :-(.

Test Plan: Display diff with no newline at end of file in new file.

Reviewers: davidreuss, epriestley

Reviewed By: davidreuss

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1911