Whether to demand confirmation of completion before exiting minibuffer.
If nil, confirmation is not required.
If the value is `confirm', the user may exit with an input that is not
a valid completion alternative, but Emacs asks for confirmation.
If the value is `confirm-after-completion', the user may exit with an
input that is not a valid completion alternative, but Emacs asks for
confirmation if the user submitted the input right after any of the
completion commands listed in `minibuffer-confirm-exit-commands'.