config: use wtree for non-repo configs (#4341)

For example, when `dvc import`ing something, dvc won't see stuff
declared in system and global configs, because GitTree doesn't see stuff
that is outside of the git repo.

Discord context:
https://discordapp.com/channels/485586884165107732/563406153334128681/740543492417126521

dvc/config.py

@@ -324,25 +324,32 @@ def load(self, validate=True):
         if not self["cache"].get("dir") and self.dvc_dir:
             self["cache"]["dir"] = os.path.join(self.dvc_dir, "cache")
 
+    def _get_tree(self, level):
+        # NOTE: this might be a GitTree, which doesn't see things outside of
+        # the repo.
+        return self.tree if level == "repo" else self.wtree
+
     def _load_config(self, level):
         filename = self.files[level]
+        tree = self._get_tree(level)
 
-        if self.tree.exists(filename, use_dvcignore=False):
-            with self.tree.open(filename) as fobj:
+        if tree.exists(filename, use_dvcignore=False):
+            with tree.open(filename) as fobj:
                 conf_obj = configobj.ConfigObj(fobj)
         else:
             conf_obj = configobj.ConfigObj()
         return _parse_remotes(_lower_keys(conf_obj.dict()))
 
     def _save_config(self, level, conf_dict):
         filename = self.files[level]
+        tree = self._get_tree(level)
 
         logger.debug(f"Writing '{filename}'.")
 
-        self.tree.makedirs(os.path.dirname(filename))
+        tree.makedirs(os.path.dirname(filename))
 
         config = configobj.ConfigObj(_pack_remotes(conf_dict))
-        with self.tree.open(filename, "wb") as fobj:
+        with tree.open(filename, "wb") as fobj:
             config.write(fobj)
         config.filename = filename
 

tests/unit/test_config.py

@@ -3,6 +3,7 @@
 import pytest
 
 from dvc.config import Config
+from dvc.tree.local import LocalTree
 
 
 @pytest.mark.parametrize(
@@ -16,3 +17,19 @@
 )
 def test_to_relpath(path, expected):
     assert Config._to_relpath(os.path.join(".", "config"), path) == expected
+
+
+def test_get_tree(tmp_dir, scm):
+    tmp_dir.scm_gen("foo", "foo", commit="add foo")
+
+    tree = scm.get_tree("master")
+    config = Config(tree=tree)
+
+    assert config.tree == tree
+    assert config.wtree != tree
+    assert isinstance(config.wtree, LocalTree)
+
+    assert config._get_tree("repo") == tree
+    assert config._get_tree("local") == config.wtree
+    assert config._get_tree("global") == config.wtree
+    assert config._get_tree("system") == config.wtree