dirstate: replace _uncommitdirstate() by core's movedirstate()

IIRC, when _uncommitdirstate() was upstreamed, it got cleaned up a
lot. Let's switch to the simpler upstream version.

This fixes the brokeness in `hg uncommit` demonstrated by the previous
patch. The problem was that _uncommitdirstate() had not been properly
updated to support uncommitting to another commit. If you replace
`oldctx.p1()` by `ctx` there, you would get this same diff as from
this patch (the `interactive` code path had been fixed this way
already).