bug#5310: vc-revert modifies window configuration

[martin rudalics]

> When calling vc-revert on a window configuration other than the default, it
> is likely to change as a result.
>
> To reproduce:
> emacs -q
> C-x 3
> In one window, open a file under version control and modify it
> M-x vc-revert
> confirm the reversion
>
> At this point, the second window will be closed.

I checked in a slightly different fix.  Please have a look.

Thanks, martin