SCM is used for both reading and writing operations, but in the former case -
as filesystem and in the latter - as actual scm/git. So we should distinguish
those and use fs directly, which follows the overall philosophy of being able
to use Repo on any fs.
Keeping `rev=` option for `Repo` because it is a very convenient way to get
repo instance from local path but for particular branch/tag/etc.
Pre-requisite for git/dvc/repofs refactorings.