bug#43572: Feature request: make it possible to choose whether the first lines of the minibuffer should be displayed instead of the last ones

[Gregory Heytings via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

>>> Binding the variable inside the minibuffer-setup-hook will affect all 
>>> the subsequent calls to resize_mini_window, until the next call to 
>>> read-from-minibuffer resets it, which may not be what the Lisp program 
>>> wants, and could have unintended consequences.
>>
>> I can't think of such unintended consequences.  In the use case of 
>> displaying completion candidates, this (the fact that it affects all 
>> successive calls to resize_mini_window) is indeed what is wanted.
>
> Well, I _can_ think of such consequences.  As I said, resize_mini_window 
> is called in many situations that don't involve completion, so setting 
> that variable to affect all of them is a bad idea.  We need something 
> more fine-grained if we want to implement such a feature.
>

I believe I have dealt with these consequences (or at least with some of 
them) in the update patch I just sent.  Feel free to tell me if there are 
other cases in which resize_mini_window() is called with a wrong value for 
start_display_at_beginning_of_minibuffer.  It is now saved by 
read_minibuf(), and reset by read_minibuf() and read_minibuf_unwind().