Let ediff '=' compare against ancestor buffer (Bug#11320)

* lisp/vc/ediff-util.el (ediff-inferior-compare-regions): Ask user
whether to compare against the ancestor or merge buffer.  Use
read-multiple-choice for A vs B buffer query.

lisp/vc/ediff-util.el

@@ -3546,25 +3546,19 @@ Ediff Control Panel to restore highlighting."
     (ediff-paint-background-regions 'unhighlight)
 
     (cond ((ediff-merge-job)
-	   (setq bufB ediff-buffer-C)
 	   ;; ask which buffer to compare to the merge buffer
-	   (while (cond ((eq answer ?A)
-			 (setq bufA ediff-buffer-A
-			       possibilities '(?B))
-			 nil)
-			((eq answer ?B)
-			 (setq bufA ediff-buffer-B
-			       possibilities '(?A))
-			 nil)
-			((equal answer ""))
-			(t (beep 1)
-			   (message "Valid values are A or B")
-			   (sit-for 2)
-			   t))
-	     (let ((cursor-in-echo-area t))
-	       (message
-		"Which buffer to compare to the merge buffer (A or B)? ")
-	       (setq answer (capitalize (read-char-exclusive))))))
+	   (setq answer (read-multiple-choice
+                         "Which buffer to compare?"
+                         '((?a "A")
+                           (?b "B"))))
+           (if (eq (car answer) ?a)
+               (setq bufA ediff-buffer-A)
+             (setq bufA ediff-buffer-B))
+           (setq bufB (if (and ediff-ancestor-buffer
+                               (y-or-n-p (format "Compare %s against ancestor buffer?"
+                                                 (cadr answer))))
+                          ediff-ancestor-buffer
+                        ediff-buffer-C)))
 
 	  ((ediff-3way-comparison-job)
 	   ;; ask which two buffers to compare