Fix bug#20233, do not merge with master

* lisp/net/tramp.el (tramp-handle-unhandled-file-name-directory):
Return nil when required by the spec.  (Bug#20233)

lisp/ChangeLog

+2015-04-01  Michael Albinus  <michael.albinus@gmx.de>
+
+	* net/tramp.el (tramp-handle-unhandled-file-name-directory):
+	Return nil when required by the spec.  (Bug#20233)
+
 2015-04-01  Nicolas Petton  <nicolas@petton.fr>
 
 	* Version 24.5 released.

lisp/net/tramp.el

@@ -3355,10 +3355,11 @@ User is always nil."
 
 (defun tramp-handle-unhandled-file-name-directory (_filename)
   "Like `unhandled-file-name-directory' for Tramp files."
-  ;; With Emacs 23, we could simply return `nil'.  But we must keep it
-  ;; for backward compatibility.  "~/" cannot be returned, because
-  ;; there might be machines without a HOME directory (like hydra).
-  "/")
+  ;; Starting with Emacs 23, we must simply return `nil'.  But we must
+  ;; keep backward compatibility, also with XEmacs.  "~/" cannot be
+  ;; returned, because there might be machines without a HOME
+  ;; directory (like hydra).
+  (and (< emacs-major-version 23) "/"))
 
 (defun tramp-handle-set-visited-file-modtime (&optional time-list)
   "Like `set-visited-file-modtime' for Tramp files."