bug#1256: Race condition in vc-diff

[Chong Yidong]

>    If you visit this file and do "C-x v =" in a single-window frame
> that is more than about 30 lines tall, and if the backend diff command
> is fast enough, the diff window will remain half the size of the
> screen, rather than being shrunk to fit the buffer.  This is because
> vc-exec-after finds that the buffer process has already finished, and
> runs vc-diff-finish before the diff buffer has been made visible.  In
> that case, vc-diff-finish assumes that the user has already buried it.
>
>    I am not a big fan of this shrink-wrapping, but I do think vc-diff
> ought to behave consistently (and it has worked this way for quite a
> while now).  The attached patch is sufficient to fix it.

I committed the patch to trunk.  Thanks.