Use "internal" smoothing for code diffs in Arcanist

Summary: Ref T7643. This moves the prefix/suffix smoothing behavior back to the old one, which seems better for code diffs.

Test Plan: `arc unit --everything`

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T7643

Differential Revision: https://secure.phabricator.com/D16074

src/difference/ArcanistDiffUtils.php

@@ -151,7 +151,7 @@ final class ArcanistDiffUtils extends Phobject {
       ->setReplaceCost(2)
       ->setMaximumLength($max)
       ->setSequences($ov, $nv)
-      ->setApplySmoothing(true)
+      ->setApplySmoothing(PhutilEditDistanceMatrix::SMOOTHING_INTERNAL)
       ->getEditString();
   }