patchCallback(patch, before, after, local, changes) {
    this._objectUpdate(patch.diffs, before, after, local, changes)
  }
Called by an Automerge document when `patch` is applied. `before` is the
state of the document before the patch, and `after` is the state after
applying it. `local` is true if the update is a result of locally calling
`Automerge.change()`, and false otherwise. `changes` is an array of
changes that were applied to the document (as Uint8Arrays).