bug#39822: 27.0.90; Cannot set *Completions* buffer height using display-buffer-alist

[Juri Linkov]

>> LGTM.  'body-function' needs an alist entry in the manual then.
>
> Pushed to master with updating the manual.
>
> Please check if this report could be closed now.

Actually it broke choose-completion invoked in *Completions* displayed
from the minibuffer, because choose-completion expects 
completion-reference-buffer
to be the original buffer (should be the minibuffer in this case),
but it was *Completions*.  This means that display-completion-list
should be run in the original buffer.

Now fixed in master.