bug#62700: 29.0.60; minibuffer-{previous,next,choose}-completion behave unintuitively when point is not at end of buffer

[Juri Linkov]

>> > Changing the API will definitely cause problems with 
>> > backwards-compatibility.
>> > But maybe you could find a simple heuristic that would decide what 
>> > base-suffix
>> > to set in minibuffer-completion-help?  Then no API changes will be needed.
>>
>> Thank you for the guidance and suggestion.
>>
>> Here's one heuristic which works decently well:
>
> If this is for master, I'm fine with such changes.  But if you intend
> to request installing this on emacs-29, then I will object making
> non-trivial changes in any code that is not specific to the M-<UP> and

Actually, a change for base-suffix in minibuffer-completion-help
is a trivial change.  What counts as a non-trivial change would be
changing the API in completion-all-completions.

> M-<DOWN> bindings that are new in Emacs 29.  I don't want to risk any
> regressions in general-purpose completion code at this late stage.

These changes are specific to the M-<UP> and M-<DOWN> bindings:
completion-use-base-affixes is nil, and it's let-bound to t
in M-<UP> (minibuffer-previous-completion) and M-<DOWN>
(minibuffer-next-completion).