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

[Carlos Pita <carlosjosepita@gmail.com>]

> > *Completions* buffer: one won't do it, exactly two won't always do
> > it (both of them may complete something), so there must be a
> > sequence of at least two Tabs and the last one must be unable to
> > complete.
>
> The 2 TABs are necessary if the partial completion is not what you
> want.  Emacs cannot know that, only you, the user, can.

But then, by the same token, why is this not true of step 5?

There I'm in ~/Desktop coming from a previous completing Tab, a "not
unique" message was not yet shown, I might press Tab again and get
another completion (perhaps there is a single subdirectory of
~/Desktop, perhaps all subdirectories share a common prefix) or not
(by far the most likely case). I press Tab and get "not unique" plus
candidates.

Suppose I now abort the process and do `M-x cd ~/Desktop` and then
enter the find-file completion again, I'm in the same situation, yet I
press Tab and I get "not unique" and I have to press Tab again in
order to get the list of candidates. Why should the behavior differ?
The only difference is that current != previous command, but that
seems hardly relevant to the matter.