This is my version of Augie's D8568. It makes it so we don't touch the
mergestate on disk when using an in-memory context.
The reason that I want this is not the same as the reason that Augie
write his patch (though I agree with that reason too). My hope is to
make in-memory rebase not fall back to on-disk rebase when there are
conflict. I plan to do that by adding a
overlayworkingctx.reflect_in_workingcopy(). The idea is that that
will update the working copy, the dirstate and the mergestate as