Update to delta method in diff match patch class to stop splitting surrogate pairs

Author: theck13

Simperium/src/main/java/name/fraser/neil/plaintext/diff_match_patch.java

@@ -1429,7 +1429,22 @@ public int diff_levenshtein(LinkedList<Diff> diffs) {
    */
   public String diff_toDelta(LinkedList<Diff> diffs) {
     StringBuilder text = new StringBuilder();
+    char lastEnd = 0;
+    boolean isFirst = true;
     for (Diff aDiff : diffs) {
+      char thisTop = aDiff.text.charAt(0);
+      char thisEnd = aDiff.text.charAt(aDiff.text.length() - 1);
+      if (Character.isHighSurrogate(thisEnd)) {
+        aDiff.text = aDiff.text.substring(0, aDiff.text.length() - 1);
+      }
+      if (! isFirst && Character.isHighSurrogate(lastEnd) && Character.isLowSurrogate(thisTop)) {
+        aDiff.text = lastEnd + aDiff.text;
+      }
+      isFirst = false;
+      lastEnd = thisEnd;
+      if ( aDiff.text.isEmpty() ) {
+        continue;
+      }
       switch (aDiff.operation) {
       case INSERT:
         try {