Deal with mw:DiffMarker markers in preProcessDOM.

* mw:DiffMarkers split up white-space and comment runs which
  messes with selser wanting separators in a single chunk.
* To prevent that, preprocessDOM normalizes the DOM by floating
  diff markers before all preceding ws/comment nodes.

Change-Id: I82cd2fd3f031a42bbf15961a9735796ac42694c6