(ido-max-directory-size): New defcustom.

(ido-max-directory-size): New defcustom.
(ido-decorations): Add "too big" element.
(ido-directory-too-big): New dynamic var.
(ido-may-cache-directory): Don't cache big directories.
(ido-directory-too-big-p): New defun.
(ido-set-current-directory): Update ido-directory-too-big.
(ido-read-internal): Make empty ido-cur-item if too-big.
(ido-buffer-internal): Use ido-read-internal directly instead of
ido-read-buffer.
(ido-file-internal): Init ido-directory-too-big.
(ido-complete): <TAB> If ido-directory-too-big is set, clear it,
and redo completion with full list.
(ido-toggle-ignore): <C-a> If ido-directory-too-big is set, clear
it, and show completions.
(ido-all-completions): Let bind ido-directory-too-big to nil.
(ido-exhibit): Handle ido-directory-too-big.
(ido-read-buffer): Handle fallback to read-buffer.  Init
ido-directory-too-big.
(ido-read-file-name, ido-read-directory-name, ido-completing-read):
Init ido-directory-too-big.