Make ediff-union into obsolete alias for seq-union

* lisp/emacs-lisp/seq.el (seq-union): Autoload.
* lisp/vc/ediff-util.el (ediff-union): Make into obsolete function
alias for 'seq-union'.
* lisp/vc/ediff-mult.el (ediff-intersect-directories): Update single
caller.

lisp/emacs-lisp/seq.el

@@ -467,6 +467,7 @@ negative integer or 0, nil is returned."
         (setq sequence (seq-drop sequence n)))
       (nreverse result))))
 
+;;;###autoload
 (cl-defgeneric seq-union (sequence1 sequence2 &optional testfn)
   "Return a list of all elements that appear in either SEQUENCE1 or SEQUENCE2.
 Equality is defined by TESTFN if non-nil or by `equal' if nil."

lisp/vc/ediff-mult.el

@@ -623,9 +623,9 @@ behavior."
 
     ;; compute difference list
     (setq difflist (ediff-set-difference
-		    (ediff-union (ediff-union lis1 lis2 #'string=)
-				 lis3
-				 #'string=)
+                    (seq-union (seq-union lis1 lis2 #'string=)
+                               lis3
+                               #'string=)
 		    common
 		    #'string=)
 	  difflist (delete "."  difflist)

lisp/vc/ediff-util.el

@@ -4161,20 +4161,6 @@ Mail anyway? (y or n) ")
       (setq lis1 (cdr lis1)))
     (cdr result)))
 
-
-;; eliminates duplicates using comparison-func
-(defun ediff-union (lis1 lis2 comparison-func)
-  (let ((result (list 'a)))
-    (while lis1
-      (or (ediff-member (car lis1) (cdr result) comparison-func)
-	  (nconc result (list (car lis1))))
-      (setq lis1 (cdr lis1)))
-    (while lis2
-      (or (ediff-member (car lis2) (cdr result) comparison-func)
-	  (nconc result (list (car lis2))))
-      (setq lis2 (cdr lis2)))
-    (cdr result)))
-
 ;; eliminates duplicates using comparison-func
 (defun ediff-set-difference (lis1 lis2 comparison-func)
   (let ((result (list 'a)))
@@ -4187,6 +4173,7 @@ Mail anyway? (y or n) ")
 
 (define-obsolete-function-alias 'ediff-add-to-history #'add-to-history "27.1")
 (define-obsolete-function-alias 'ediff-copy-list #'copy-sequence "28.1")
+(define-obsolete-function-alias 'ediff-union #'seq-union "28.1")
 
 (run-hooks 'ediff-load-hook)