Previously, when rolling back a transaction, some users could be confused
between the level to which the store is rolled back, and the new parents
of the working directory.
$ hg rollback
rolling back to revision 4 (undo commit)
With this change:
$ hg rollback
repository tip rolled back to tip revision 4 (undo commit)
working directory now based on revision 2 and 1
So now the user can realize that the store has been rolled back to an older
tip, but also that the working directory may not on the tip (here we are
rolling back the merge of the heads 2 and 1)
