Make arc diff --background 1 default and disable it on Windows

Summary:
I am using it for about a month.
It works even in Facebook www.

Test Plan:

$ arc diff --background 1 # hundreds of times

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3495