Emacs VC support for Darcs

#2Emacs 26.1 complains about duplicate &optional on vc-darcs-diff

vc-darcs.el :: line 747

(defun vc-darcs-diff (file &optional rev1 rev2 buffer &optional async)

should be:

(defun vc-darcs-diff (file &optional rev1 rev2 buffer async)

I changed that in a fork.