dirstate: explain why appending instead of os.path.join() is safe

mercurial/dirstate.py

@@ -101,6 +101,7 b' class dirstate(object):'
 
     def _join(self, f):
         # much faster than os.path.join()
+        # it's safe because f is always a relative path
         return self._rootdir + f
 
     def flagfunc(self, fallback):