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