Phabricator builds up the replacement commits and mapping in a single
transaction, and then finalizes everything once the commits have been rewritten.
That's too late when trying to update the messages for those commits.
I'm a little concerned that this isn't a generic enough interface, since it
doesn't mimic the list of list return of obsutil.successorssets(). But this
is the type of mapping that phabricator maintains, and I don't think the methods
that would be interested in calling this need to worry about split and
divergence. We can fix that later if the need arises.