Arcanist - potentially fix some Windows badness

Summary: Fixes T5914, maybs.

Test Plan: will ask users in T5914

Reviewers: chad, epriestley

Reviewed By: chad, epriestley

Subscribers: chad, bluehawk, epriestley, Korvin

Maniphest Tasks: T5914

Differential Revision: https://secure.phabricator.com/D10333