(vc-revert-buffer): Handle empty diff properly.
(vc-version-backup-file): New function.
(vc-checkout): Create a version backup if necessary.
(vc-checkin): If a version backup file exists, delete it.
(vc-version-diff): Diff locally using version backups, if available.
(vc-revert-file): If there's a version backup, revert locally.
(vc-transfer-file): Use version backup for base version, if
available. If not, ask for confirmation whether to get it from the
server. Update mode line before check-in.