git: Move LocalRepository implementation into its own package

The LocalRepository implementation has grown quite a lot up to a point where it's warrented to have its own package. So let's move it into internal/git/localrepo, similar to the already existing internal/git/remoterepo package.

There's some additional refactorings in this MR which make LocalRepository and its config/remote interfaces more self-contained. I think those are the actual nice thing about this MR.