Support interactive editing with Windows editors

Summary: Currently, output is generated with *nix newlines (`\n`) but this doesn't work in (for example) notepad. (and oh, Notepad blocks just fine)

Test Plan: run `arc diff` with and without the patch. See a mess before, and beauty after

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9504