typing: add type annotations to the dirstate classes The basic procedure here was to use `merge-pyi` to merge the `git/dirstate.pyi` file in (after renaming the interface class to match), cleaning up the import statement mess, and then repeating the procedure for `mercurial/dirstate.pyi`. Surprisingly, git's dirstate had more hints inferred in its *.pyi file. After that, it was a manual examination of each method in the interface, and how they were implemented in the core and git classes to verify what was inferred by pytype, and fill in the missing gaps. Since this involved jumping around between three different files, I applied the same type info to all three at the same time. Complex types I rolled up into type aliases in the interface module, and used that as needed. That way if it changes, there's one place to edit. There are some hints still missing, and some documentation that doesn't match the signatures. They should all be marked with TODOs. There are also a bunch of methods on the core class that aren't on the Protocol class that seem like maybe they should be (like `set_tracked()`). There are even more methods missing from the git class. But that's a project for another time.