repo: use fs instead of scm

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.

tests/unit/repo/test_repo.py

@@ -120,9 +120,9 @@ def test_branch_config(tmp_dir, scm):
     scm.checkout("master")
 
     with pytest.raises(NotDvcRepoError):
-        Repo(scm=scm, rev="master").close()
+        Repo(rev="master").close()
 
-    dvc = Repo(scm=scm, rev="branch")
+    dvc = Repo(rev="branch")
     try:
         assert dvc.config["remote"]["branch"]["url"] == "/some/path"
     finally: