Emacs upstream removed the variable in https://github.com/emacs-mirror/emacs/commit/ed6ce56
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(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=21969
vc-darcs-program-name file
"diff"
(append from to arguments))))