Allow searching interactively over completions in `M-x'

* lisp/simple.el (read-extended-command): Allow doing interactive
searches over the completions (bug#12490).  This restores the
behaviour from Emacs 23 that was lost in Emacs 24.

etc/NEWS

@@ -819,6 +819,12 @@ window after starting).  This variable defaults to nil.
 
 ** Miscellaneous
 
+---
+*** 'C-s' in 'M-x' now searches over completions again.
+In Emacs 23, typing 'M-x' ('read-extended-command') and then 'C-s' (to
+do an interactive search) would search over possible completions.
+This was lost in Emacs 24, but is now back again.
+
 ---
 *** 'M-x report-emacs-bug' will no longer include "Recent messages" section.
 These were taken from the "*Messages*" buffer, and may inadvertently

lisp/simple.el

@@ -1847,9 +1847,15 @@ to get different commands to edit and resubmit."
 	     (lambda ()
 	       ;; Get a command name at point in the original buffer
 	       ;; to propose it after M-n.
-	       (with-current-buffer (window-buffer (minibuffer-selected-window))
-		 (and (commandp (function-called-at-point))
-		      (format "%S" (function-called-at-point)))))))
+	       (let ((def (with-current-buffer
+			      (window-buffer (minibuffer-selected-window))
+			    (and (commandp (function-called-at-point))
+				 (format "%S" (function-called-at-point)))))
+		     (all (sort (minibuffer-default-add-completions)
+				(lambda (a b) (string< a b)))))
+		 (if def
+		     (cons def (delete def all))
+		   all)))))
     ;; Read a string, completing from and restricting to the set of
     ;; all defined commands.  Don't provide any initial input.
     ;; Save the command read on the extended-command history list.