bug#17809: 24.4.50; Completions display

[Juri Linkov]

When the width of the window with *Completions* is more
than the default 80 columns, it leaves too much empty space.

So it's better first to display an empty buffer,
and then fill it with the contents using its final width,
but currently this is not supported.

In `with-output-to-temp-buffer' I tried to reverse
the order of lines:

  (internal-temp-output-buffer-show ,buf)
  (progn ,@body)

and it seems to work, but the problem is that
an alist of display-buffer with

  (window-height . fit-window-to-buffer)

is performed on the empty buffer, and reduces it to 3 lines.
Then later (progn ,@body) fills it with completions,
but the window height remains 3 lines.

I see no way to perform these steps using the current
display-buffer actions:

1. display the buffer
2. fill it
3. call action alists with fit-window-to-buffer