fixup! squash! squash! Make more use of get_dirs_from_args

Author: blueyed

_pytest/config.py

@@ -1147,7 +1147,12 @@ def determine_setup(inifile, args):
                 if rootdir is None:
                     cwd = py.path.local()
                     rootdir = get_common_ancestor([cwd, ancestor])
-                    if rootdir.dirname == '' or rootdir.dirname == rootdir:
+
+                    def is_at_root(d):
+                        import os
+                        return os.path.splitdrive(d) == os.sep
+
+                    if is_at_root(rootdir):
                         rootdir = ancestor
     return rootdir, inifile, inicfg or {}