(vc-resynch-buffer): New function.

(vc-resynch-buffer): New function.
(vc-locked-example): Renamed to vc-snapshot-precondition. It now also
checks whether any of the files are visited.
(vc-retrieve-snapshot): If any files are visited, ask whether to
revert their buffers. Use vc-backend-checkout and vc-resynch-buffer
to do that, instead of vc-checkout.

(vc-backend-checkout): Adjust default-directory so that the
checked-out file goes to the right place.