Step towards working arc diff --background 1 on Windows

Summary:
Running a.php from command line doesn't work on Windows, we need to run php a.php.
This shouldn't break other OSes.

Test Plan:

$ arc diff --background 1

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3544