bug#13045: 24.2.90; dired-guess-shell-command: default in prompt after colon

[Christopher Schmidt]

    emacs -q
    C-x C-f /tmp RET
    M-x load-library RET dired-x RET
    M-! touch rms.tar RET
    g
    # Move point to rms.tar
    !

The prompt says

    ! on rms.tar: {3 guesses} [tar xvf]

I think it should say

    ! on rms.tar {3 guesses} [tar xvf]:

This is consistent with most other commands that insert the default in
the prompt.

The code that generates the prompt is in dired-guess-shell-command.

I think it is a good idea to add a new magic options to Emacs which
will, if customised, unconditionally add the default value, if there is
one, to the minibuffer prompt.  WDYT?

        Christopher