Emacs upstream removed the variable in https://github.com/emacs-mirror/emacs/commit/ed6ce56
MX4R6WKHIKYB7244BG2MWV6PAGPNXJCOTQQWAFQPJCNKK6HXK3QAC (let ((status (apply #'vc-do-command (or buffer "*vc-diff*")(if async 'async 1)vc-darcs-program-name file"diff"(append from to arguments))))(if async 1 status))))
(apply #'vc-do-command (or buffer "*vc-diff*")nil ;; https://debbugs.gnu.org/cgi/bugreport.cgi?bug=21969vc-darcs-program-name file"diff"(append from to arguments))))