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."