[PATCH 01 of 10 layering] dirstateguard: move to new module so I can break some layering violations

[timeless]

Yuya Nishihara wrote:
> Is copy-and-remove a preferred way for this kind of refactoring? I have
> an unsent patch that splits revset.py, so I want to know which is better.

Augie Fackler wrote:
> I'd prefer it slightly, because it makes the blame history a little bit more intact in the long run.

Me too.