> We could make sure to call the function with the current number of
> arguments when a patch-buffer is not used, and with the additional one when
> it is used. Which would automatically force an abort when a backend does
> not support this feature.

This is fine, so this patch does this:

         (if patch-string
             (vc-call-backend backend 'checkin files comment rev patch-string)
           (vc-call-backend backend 'checkin files comment rev))

Also I added the new arg ‘patch-string’ to vc-checkin and vc-start-logentry.
These are not API calls, so they could use a buffer-local variables instead
of args, but I'm not sure about this less important technical detail.