Populate rev_content_model when a move causes default content model to change
If the default content model changes due to a page move, a NULL in
rev_content_model will now reference the new default, which is not how
the revision was serialized, causing an exception to be thrown.
This was caused by a5bc9f49cd01e, since previously NULL would use the
correct, but techncially inaccurate, page_content_model.