The RevisionSlotsUpdate interface represents a change to a pages slots,
as applied by an edit.
This also introduces RevisionSlots::hasSameContent and pulls up
getTouchedSlots() and getInheritedSlots() from MutableRevisionStore
to RevisionStore, in preparation of using these classes in the
refactoring of WikiPage::doEditContent and friends.
Bug: T174038
Change-Id: Idb0ef885b343a76137b640fdfc1bf36104b00895
