bug#6870: Make vc-root-diff work in more modes

[Chong Yidong]

Stefan Monnier <monnier@iro.umontreal.ca> writes:

>> What about all buffers that have file names?  A la
>
>> (and (buffer-file-name)
>>      (vc-responsible-backend default-directory))
>
> That'd be fine, yes.

But this might be confusing, since this check would not trigger for
non-file buffers generated from those file buffers.

i.e., if you have a buffer with a file name that is not handled by VC,
vc-root-diff would work for that buffer but not for a diff file
generated from that buffer.

Whereas if a buffer with a file name *is* handled by VC, vc-root-diff
would work for both that buffer and a diff file generated for that
buffer.