Use EditDistanceMatrix diff smoothing in Arcanist

Summary:
Ref T7643. We've done the smoothing from D16068 for a long time, it just wasn't part of EditDistanceMatrix.

Since it now is, call it instead of keeping a copy of the logic around.

Previously, the unit tests were testing un-smoothed diffs. Just have them test smoothed diffs instead, since nothing actually uses unsmoothed diffs.

Test Plan: Pasted a raw diff into the web UI, got a sensible inline diff for it.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T7643

Differential Revision: https://secure.phabricator.com/D16069