Use "--whitespace nowarn" in arc patch to respect trailing whitespace

Summary: Fixes T10008. Git tries to fix some issues by default (apparently? empirically; not consistent with documentation, I think?), but patches from arc patch are "always" accurate (disregarding other bugs we might have -- basically, they haven't been emailed or copy/pasted or anything like that) so we can just tell it to apply the patch exactly as-is.

Test Plan:

Reviewers: chad, joshuaspence

Reviewed By: chad, joshuaspence

Maniphest Tasks: T10008

Differential Revision: https://secure.phabricator.com/D14816