bug#19099: Diff options with vc-git

[Glenn Morris]

Eli Zaretskii wrote:

> You can make an alias as described here:
>
>   
> http://stackoverflow.com/questions/3129514/how-to-make-git-diff-create-a-context-format-diff
>
> and then invoke that alias instead of "diff".

Git aliases cannot override built-in git commands. (By design, wontfix, etc.)
So I cannot make "git diff" do what I want.

If you think otherwise, please give me a concrete example of how I can
configure Emacs to do what I want.