Load contents of shielded files with lots of changes

Summary:
We need $this->old and $this->new in renderShield().

Broken since D2358.

Test Plan: Loaded contents of file with lots of changes.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3080