fixup! Fix determining rootdir from common_ancestor

Author: blueyed

_pytest/config.py

@@ -1140,7 +1140,8 @@ def determine_setup(inifile, args):
                     dirs, ["pytest.ini", "tox.ini", "setup.cfg"])
                 if rootdir is None:
                     rootdir = get_common_ancestor([py.path.local(), ancestor])
-                    if os.path.splitdrive(str(rootdir))[1] == os.sep:
+                    is_fs_root = os.path.splitdrive(str(rootdir))[1] == os.sep
+                    if is_fs_root:
                         rootdir = ancestor
     return rootdir, inifile, inicfg or {}