bug#51650: Autocomplete: first Tab should show *Completions* buffer

[Eli Zaretskii]

> From: Carlos Pita <carlosjosepita@gmail.com>
> Date: Sun, 7 Nov 2021 16:34:29 -0300
> Cc: 51650@debbugs.gnu.org
> 
> So far so good, but I'm comparing the second Tab in scenario #2 to the
> first Tab in scenario #1 because they are both in states characterized
> by:
> 
> a. a valid response
> b. a non-unique completion
> c. no information yet communicated to the user about b

But the difference is that until the first TAB the user cannot know
whether there is any completion.  Only after Emacs doesn't complete is
that fact known.  And that's the reason for the difference in
behavior.