bug#591: 23.0.60; lisp-complete-symbol erases extra text

[Chong Yidong]

> emacs -Q
> 
> In *scratch*, type this, and leave cursor after the final `for':
> 
> format-decode-buffer
> forward-char
> 
> for
> 
> Then hit `M-TAB'. Choose one of the completions using mouse-2 in 
> *Completions*.
> 
> Only the final `for' should be completed, but instead all of the text
> in the buffer is replaced by the chosen completion.

Hi Stefan,

This bug was introduced by your patch:

2008-04-13  Stefan Monnier  <monnier@iro.umontreal.ca>

 minibuffer.el (display-completion-list):
 Handle all-completions's new base-size info to set completion-base-size.

    (with-current-buffer standard-output
      ...
        (insert "Possible completions are:\n")
        (let ((last (last completions)))
          ;; Get the base-size from the tail of the list.
          (set (make-local-variable 'completion-base-size) (or (cdr last) 0))
          (setcdr last nil)) ;Make completions a properly nil-terminated list.
        (completion--insert-strings completions))))

This sets completion-base-size to 0, which causes the completions buffer
to delete everything in the Lisp buffer when you make a selection.

I don't understand why we need the (completely undocumented) hack
introduced here, where the cdr of the last item on the completions list
gives completion-base-size.  Does anything else in Emacs depend on this?
Why not simply add a new optional argument to display-completion-list?