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

[Carlos Pita]

> 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.

First of all, I'm assuming:

- The initial value is taken from the current working directory so
it's always a valid completion.
- TAB never completes across the directory boundary.

Let's consider:

- TAB on ~/ (i.e. first TAB in #1)
- TAB on ~/Desktop/ after TAB-completing from ~/Desk (i.e.
second TAB in #2)
- TAB on ~/Desktop/ with cwd = ~/Desktop/ (i.e. first TAB in #3).

AFAICS these statements are true at the beginning (before pressing
TAB) of all cases:

- The user knows the current completion is valid.
- The user cannot know whether there is any other completion with the
same prefix.

You seem to be saying that the TAB that produced ~/Desktop/ from
~/Desk is adding some worthy information about the contents of
~/Desktop, and therefore about possible completions, that
wouldn't be there if the user, say, launched emacs from ~/Desktop
as in #3. But, for the life of me, I cannot see it :(