Fix two minor Differential issues

Summary:
  - D6966 accidentally reversed the order of `$diffs`. Reverse it back.
  - The new policy header stuff returns `array(icon, text)` but gets `strlen()`'d by a caller. Silence that warning for now.

Test Plan: Created a revision with several diffs. Saw them in the right order; saw no warning on the diff attach screen.

Reviewers: chad, btrahan

Reviewed By: chad

CC: aran, mbishopim3

Differential Revision: https://secure.phabricator.com/D7023