rebase: abort in-mem rebase if there's a dirty merge state
In-memory merge uses the on-disk merge state, so we should not allow
it run in-memory merge when the merge state is not clean. We should
probably not use the on-disk merge state when running in-memory merge,
but chaning that is not suitable for the stable branch.
Differential Revision: https://phab.mercurial-scm.org/D5357
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