merge: use in-memory mergestate when using in-memory context 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 necessary. Differential Revision: https://phab.mercurial-scm.org/D9040