Add getHistoryGeneration() to CodeMirror.js.

Author: JuBan1

lib/codemirror.js

@@ -6216,6 +6216,9 @@ Doc.prototype = createObj(BranchChunk.prototype, {
   isClean: function (gen) {
     return this.history.generation == (gen || this.cleanGeneration)
   },
+  getHistoryGeneration: function() {
+  	return this.history.generation;
+  },
 
   getHistory: function() {
     return {done: copyHistoryArray(this.history.done),