> Another solution for making sure the current backend supports the new
> feature is to add a new backend function. Call it 'checkin-patch' or
> `checkin-diff`.
>
> vc-git-checkin and vc-git-checking patch can share some implementation, so
> most of code duplication can be avoided.
>
> But when a backend does not support this, the user will see a more-or-less
> understandable error like "not supported".

So a new patch adds a new backend function 'checkin-patch':