vc-diff-switches

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

nil

Documentation:
A string or list of strings specifying switches for diff under VC.
When running diff under a given BACKEND, VC uses the first
non-nil value of `vc-BACKEND-diff-switches', `vc-diff-switches',
and `diff-switches', in that order. Since nil means to check the
next variable in the sequence, either of the first two may use
the value t to mean no switches at all. `vc-diff-switches'
should contain switches that are specific to version control, but
not specific to any particular backend.

You can customize this variable.

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