use git rev-parse to look for config file

Author: bdauvergne

git/repo/base.py

@@ -431,7 +431,7 @@ def _get_config_path(self, config_level):
         elif config_level == "global":
             return osp.normpath(osp.expanduser("~/.gitconfig"))
         elif config_level == "repository":
-            return osp.normpath(osp.join(self._common_dir or self.git_dir, "config"))
+            return osp.normpath(self.git.rev_parse("config", git_path=True))
 
         raise ValueError("Invalid configuration level: %r" % config_level)