Don't bind M-= in Dired.

* lisp/dired.el (dired-mode-map): Don't bind M-=.

* lisp/dired-aux.el (dired-diff): Use backup file as default.

etc/NEWS

@@ -423,6 +423,13 @@ channel keys found, if any.
 if the command ends in `;' (when operating on multiple files).
 Otherwise, it executes the command on each file in parallel.
 
+*** The minibuffer default for `=' (`dired-diff) has changed.
+It is now the backup file for the file at point, if one exists, rather
+than the file at the mark.
+
+*** `M-=' is no longer bound to `dired-backup-diff' in Dired buffers.
+The global binding for `M-=', `count-words-region' is in effect.
+
 ** Shell
 
 *** New option `async-shell-command-buffer' specifies what buffer to use

lisp/ChangeLog

+2012-09-08  Chong Yidong  <cyd@gnu.org>
+
+	* dired.el (dired-mode-map): Don't bind M-=.
+
+	* dired-aux.el (dired-diff): Use backup file as default.
+
 2012-09-08  Drew Adams  <drew.adams@oracle.com>
 	* subr.el (add-to-history): Fix delete usage (Bug#12314).

lisp/dired-aux.el

@@ -54,43 +54,30 @@ into this list; they also should call `dired-log' to log the errors.")
 ;;;###autoload
 (defun dired-diff (file &optional switches)
   "Compare file at point with file FILE using `diff'.
-FILE defaults to the file at the mark.  (That's the mark set by
-\\[set-mark-command], not by Dired's \\[dired-mark] command.)
-The prompted-for FILE is the first file given to `diff'.
+If called interactively, prompt for FILE; if the file at point
+has a backup file, use that as the default.
+
+FILE is the first file given to `diff'.
 With prefix arg, prompt for second argument SWITCHES,
 which is the string of command switches for `diff'."
   (interactive
    (let* ((current (dired-get-filename t))
-	  ;; Get the file at the mark.
-	  (file-at-mark (if (mark t)
-			    (save-excursion (goto-char (mark t))
-					    (dired-get-filename t t))))
-	  ;; Use it as default if it's not the same as the current file,
-	  ;; and the target dir is the current dir or the mark is active.
-	  (default (if (and (not (equal file-at-mark current))
-			    (or (equal (dired-dwim-target-directory)
-				       (dired-current-directory))
-				mark-active))
-		       file-at-mark))
-	  (target-dir (if default
-			  (dired-current-directory)
-			(dired-dwim-target-directory)))
-	  (defaults (dired-dwim-target-defaults (list current) target-dir)))
-     (require 'diff)
-     (list
-      (minibuffer-with-setup-hook
-	  (lambda ()
-	    (set (make-local-variable 'minibuffer-default-add-function) nil)
-	    (setq minibuffer-default defaults))
-	(read-file-name
-	 (format "Diff %s with%s: " current
-		 (if default (format " (default %s)" default) ""))
-	 target-dir default t))
-      (if current-prefix-arg
-	  (read-string "Options for diff: "
-		       (if (stringp diff-switches)
-			   diff-switches
-			 (mapconcat 'identity diff-switches " ")))))))
+	  (oldf (file-newest-backup current))
+	  (dir (if oldf (file-name-directory oldf))))
+     (list (read-file-name
+	    (format "Diff %s with%s: "
+		    (file-name-nondirectory current)
+		    (if oldf
+			(concat " (default "
+				(file-name-nondirectory oldf)
+				")")
+		      ""))
+	    dir oldf t)
+	   (if current-prefix-arg
+	       (read-string "Options for diff: "
+			    (if (stringp diff-switches)
+				diff-switches
+			      (mapconcat 'identity diff-switches " ")))))))
   (let ((current (dired-get-filename t)))
     (when (or (equal (expand-file-name file)
 		     (expand-file-name current))

lisp/dired.el

@@ -1410,7 +1410,6 @@ Do so according to the former subdir alist OLD-SUBDIR-ALIST."
     (define-key map "&" 'dired-do-async-shell-command)
     ;; Comparison commands
     (define-key map "=" 'dired-diff)
-    (define-key map "\M-=" 'dired-backup-diff)
     ;; Tree Dired commands
     (define-key map "\M-\C-?" 'dired-unmark-all-files)
     (define-key map "\M-\C-d" 'dired-tree-down)
@@ -3745,14 +3744,15 @@ Ask means pop up a menu for the user to select one of copy, move or link."
 ;;;;;;  dired-run-shell-command dired-do-shell-command dired-do-async-shell-command
 ;;;;;;  dired-clean-directory dired-do-print dired-do-touch dired-do-chown
 ;;;;;;  dired-do-chgrp dired-do-chmod dired-compare-directories dired-backup-diff
-;;;;;;  dired-diff) "dired-aux" "dired-aux.el" "9499f79f5853da0aa93d26465c7bf3a1")
+;;;;;;  dired-diff) "dired-aux" "dired-aux.el" "4b260eda371d319a6c8e8e5ec917e287")
 ;;; Generated autoloads from dired-aux.el
 
 (autoload 'dired-diff "dired-aux" "\
 Compare file at point with file FILE using `diff'.
-FILE defaults to the file at the mark.  (That's the mark set by
-\\[set-mark-command], not by Dired's \\[dired-mark] command.)
-The prompted-for FILE is the first file given to `diff'.
+If called interactively, prompt for FILE; if the file at point
+has a backup file, use that as the default.
+
+FILE is the first file given to `diff'.
 With prefix arg, prompt for second argument SWITCHES,
 which is the string of command switches for `diff'.