vc-cvs-diff-switches

vc-cvs-diff-switches is a variable defined in `vc-cvs.el'.
Its value is

nil

Documentation:
String or list of strings specifying switches for CVS diff under VC.
If nil, use the value of `vc-diff-switches'. If t, use no switches.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 21.1 of Emacs.