bug#61535: 29.0.60; choose-completion erases in-region buffer

[Juri Linkov]

>> 1. type: (with-c C-M-i
>> 2. type: M-down ... M-RET
>> 3. check that the whole buffer was deleted before a selected
>>    completion candidate was inserted to the buffer:
>>    C-h v buffer-undo-list RET
>
> Which buffer was deleted? *scratch*?  It isn't here.
>
> And what does "C-h v buffer-undo-list RET" mean?
>
> Bottom line: I don't understand what is the bug here and/or how to
> reproduce it.

The contents of the *scratch* buffer is erased and replaced
with the same text.  This fact can be confirmed by looking
at the value of 'buffer-undo-list' that contains the text
of the whole buffer as a string.  Especially this is noticeable
when using in-region completion in a large buffer.

With the patch, only the completed string is inserted.