message: bug fix; no direct call to new MergeBlock; must use makeBlock
diff: @@ -1642,8 +1642,7 @@ export class MergeTree {
  // for diagnostics
  getLongClientId: (id: number) => string;
  - constructor(public text: string, updateMarkers = false) {
  - MergeTree.blockUpdateMarkers = updateMarkers;
  + constructor(public text: string) {
  this.blockUpdateActions = MergeTree.initBlockUpdateActions;
  this.root = this.initialTextNode(this.text);
  }
  @@ -1681,7 +1680,7 @@ export class MergeTree {
  let nodeIndex = 0;
  for (let i = 0; i < nodeCount; i++) {
  let len = 0;
  - blocks[i] = new MergeBlock(0);
  + blocks[i] = this.makeBlock(0);
  for (let j = 0; j < segCap; j++) {
  if (nodeIndex < nodes.length) {
  let childIndex = this.addNode(blocks[i], nodes[nodeIndex]);
  