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
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