rebase: abort in-mem rebase if there's a dirty merge state...
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