dirstate: add missing return on platforms without exec or symlink

Differential Revision: https://phab.mercurial-scm.org/D11713

mercurial/dirstate.py

@@ -300,6 +300,8 b' class dirstate(object):'
                         return b''
                 return fallback(x)
 
+            return f
+
     @propertycache
     def _cwd(self):
         # internal config: ui.forcecwd