tests: show that in-memory rebase leaves state when working copy is dirty When in-memory rebase falls back to on-disk rebase, it checks if the working copy is dirty. If it is, it aborts the rebase. However, it leaves the rebase state on disk. I broke it in feffeb18d412 (rebase: teach in-memory rebase to not restart with on-disk rebase on conflict, 2020-09-18). Differential Revision: https://phab.mercurial-scm.org/D9508