Trim trailing whitespace from diff hints

Summary: Although these don't do any harm, they show up in my editor which is configured to highlight trailing whitespace.

Test Plan: Submitted this diff... saw no trailing whitespace.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D12369