PreparedUpdate represents an in-progress edit. It can be used to access
information about the edit from hooks that do to have access to the
PageUpdater. Ideally, the PreparedUpdate or PageUpdater would be passed
to the hook as a parameter. Handlers of legacy hooks may use
WikiPage::prepareUpdate() to access it.
Bug: T242249
Needed-By: I23324a31e06e7e6f28077085c0ade05db63e9a35
Change-Id: Id5ba40a21cc4547205adf2f3a1f725c3a69c24d8
