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