bug#12399: 24.2.50; dired-do-touch doesn't recognize empty input in minibuffer

[Juri Linkov]

> There's another way to do that: check (eq val default), since the
> returned value will be `eq' to the `default' if the minibuffer was
> empty, whereas if it comes from a non-empty minibuffer it will at best
> be `equal'.

I tried this in the following patch and it works.
There is too little space in comments to explain
this hack, so I added a bug reference.

Old (string-equal new-attribute "") remains as
a precaution against possible unexpected changes in
`dired-mark-read-string'.

=== modified file 'lisp/dired-aux.el'
--- lisp/dired-aux.el   2012-09-08 14:48:27 +0000
+++ lisp/dired-aux.el   2012-09-11 22:48:01 +0000
@@ -244,7 +244,10 @@ (defun dired-do-chxxx (attribute-name pr
                             (function dired-check-process)
                             (append
                              (list operation program)
-                             (unless (string-equal new-attribute "")
+                             (unless (or (string-equal new-attribute "")
+                                         ;; Use `eq' instead of `equal'
+                                         ;; to detect empty input (bug#12399).
+                                         (eq new-attribute default))
                                (if (eq op-symbol 'touch)
                                    (list "-t" new-attribute)
                                  (list new-attribute)))