fix : fix broken prefix and suffix diffs generation inside _addContext_

Author: michal-kurz

javascript/diff_match_patch_uncompressed.js

@@ -1776,12 +1776,23 @@ diff_match_patch.prototype.patch_addContext_ = function(patch, text) {
 
   // Add the prefix.
   var prefix = text.substring(patch.start2 - padding, patch.start2);
+  
+  if(this.isLowSurrogate(prefix[0])) {
+    prefix = text.substring(patch.start2 - padding - 1 , patch.start2);
+  }
+  
   if (prefix) {
     patch.diffs.unshift(new diff_match_patch.Diff(DIFF_EQUAL, prefix));
   }
   // Add the suffix.
   var suffix = text.substring(patch.start2 + patch.length1,
                               patch.start2 + patch.length1 + padding);
+  
+  if(this.isHighSurrogate(suffix[suffix.length-1])) {
+    suffix = text.substring(patch.start2 + patch.length1,
+                            patch.start2 + patch.length1 + padding + 1);
+  }
+  
   if (suffix) {
     patch.diffs.push(new diff_match_patch.Diff(DIFF_EQUAL, suffix));
   }