state: always use working trees

[pmrowla]

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PR forces State to always use WorkingTree rather than repo.tree (since state needs to be able to look up local cache files even if repo.tree is a brancher GitTree). Once Remote is refactored to use trees, we should also consider making State owned by the local cache and not Repo, so that state always uses the local cache working tree.

Closes #3812, closes #3857.

[efiop]

@pmrowla One thing that worries me here is that we will once again parse dvcignores in the project. Even though it is not critical, we should keep that in mind for the following patches. It probably makes sense to create self.repo.wtree and use it, that way dvcignore won't beparsed. CC @pared