Phriction - restore diff link

Summary: Fixes T6472. I was curious to see how many reports we'd get but we didn't get any I know of... Still, this is a nice convenient link.

Test Plan:
edited a wiki doc title + body. ran /bin/mail to see the body and liked what I saw

TEXT BODY
btrahan changed the title from "new title is a good thing" to "new title is a good thing asdsadsadsadsada".
btrahan edited the document content.

DOCUMENT DIFF
  http://phalanx.dev/phriction/diff/36/?l=3&r=4

DOCUMENT DETAIL
  http://phalanx.dev/w/

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6472

Differential Revision: https://secure.phabricator.com/D10850