Let sudo-command-line() do something sensible on an empty buffer.

Let sudo-command-line() do something sensible on an empty buffer.
When the buffer is empty, assume that the user wants to rerun the previous
command, prepended by 'sudo '. Also changed _sudo-command-line() to
sudo-command-line(). Per convention functions starting with underscore are
usually completion widgets (and it's really not needed here).