Use PhutilEditDistanceMatrix in ArcanistDiffUtils

Summary: Replace this old hard-coded implementation with the new vector-based, unicode-capable one.

Test Plan: Ran unit tests. Looked at revisions in Differential, using whitespace modes to bypass cache.

Reviewers: btrahan, Afaque_Hussain

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2379

Differential Revision: https://secure.phabricator.com/D6016