rebase: check for unfinished ops even when inmemory (issue6214)
When using rebase.experimental.inmemory, we should be able to work well with a
dirty working directory, but we can not reliably work if we're in the middle of
another operation (such as another rebase), as we'll potentially stomp on some
state that the other operation needs.
Differential Revision: https://phab.mercurial-scm.org/D7298
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