Don't throw exception in diff versus file removed from revision

Test Plan: Display a revision where a file was removed compared to previous
diffs, like https://secure.phabricator.com/D1724?vs=3008&id=3009.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1754