[merge addon] Don't use DMP's cleanupSemantic function

It sometimes produces invalid output.

Closes #4410

addon/merge/merge.js

@@ -571,7 +571,6 @@
   var dmp = new diff_match_patch();
   function getDiff(a, b) {
     var diff = dmp.diff_main(a, b);
-    dmp.diff_cleanupSemantic(diff);
     // The library sometimes leaves in empty parts, which confuse the algorithm
     for (var i = 0; i < diff.length; ++i) {
       var part = diff[i];