Congratulations!  You may have unearthed a bug in Ediff!

Please make a concise and accurate summary of what happened
and mail it to the address above.
-----------------------------------------------------------

ediff-patch-buffer fails if patch-buf doesn't exists.

* lisp/vc/ediff.el (ediff-patch-buffer): Added 'require-match
argument on read-buffer call.

emacs -Q:
(in the *scratch* buffer)

(with-current-buffer (generate-new-buffer "buff")
   (insert "foo bar"))

(with-current-buffer (generate-new-buffer "buff2")
   (insert "foo baz"))

;; call ediff-buffers on "buff" and "buff2" and save the
;; differences in a buffer "patch"

M-x epatch-buffer y patch RET buff ; ok
M-x epatch-buffer y patch RET buffer ; BAD
You have killed a vital Ediff buffer---you must leave Ediff now!


Emacs  : GNU Emacs 25.0.50.1 (x86_64-pc-linux-gnu, GTK+ Version 2.24.28)
  of 2015-11-07
Package: Ediff 2.81.5 of July 4, 2013