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

[sbaugh]

Eli Zaretskii <eliz@gnu.org> writes:
>> Cc: 62700@debbugs.gnu.org, Spencer Baugh <sbaugh@janestreet.com>
>> From: sbaugh@catern.com
>> Date: Fri, 07 Apr 2023 21:02:47 +0000 (UTC)
>> 
>> Juri Linkov <juri@linkov.net> writes:
>> > 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
> 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.

OK, that's no problem, this can be done by just let-binding
completion-base-affixes in minibuffer-{previous,next,choose}-completion
so that it only affects new code.  That will be a bit uglier to read so
I'll do that if this approach seems reasonable with some review.