fix whitespace mode for differentialHunkParser

Summary: I forgot to "set it" from the changeset parser.

Test Plan: made a diff with silly whitespace changes. toggled the various whitespace modes and got appropriate results

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2009

Differential Revision: https://secure.phabricator.com/D4378