just the savepoint being rolled back because later savepoints
might involved data managers that hadn't joined when the savepoint
being rolled back was created.

Now, when a data manager joins and we have savepoints, we create a
data manager savepoint for the new data manager and add the
datamanager savepoint to all previous transaction savepoints.  Note
that this data manager savepoint can be a special savepoint that just
calls abort on the data manager when it is rolled back.