Use binary_safe_diff from arcanist

Summary:
binary_safe_diff is needed in arcanist too. Moved it over to
arcanist. See D2915.

Test Plan: diffusion page rendered correctly on binary file.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D2916