Most methods on `Transaction` only need the `MutableRepo`, so it makes
for that functionality to be on the latter. That will let us update
the methods to also update the index, which would otherwise have been
harder because it would require a mutable borrow of both the view and
the index. This patch makes most current methods on `Transaction` just
delegate to `MutableRepo`. We may want to remove some of these
delegating methods later.
