bug#33992: 27.0.50; xref-find-definitions wastes too much space

[Juri Linkov]

>> As noted in bug#33870 the buffer produced by xref-find-definitions
>> (bound to ‘M-.’) has a transient nature like the *Completions* buffer.
>> Therefore it makes sense not to waste its space needlessly:
>
> It doesn't make sense to review this patch until the patch that fixes 33870
> has been produced.

Since the patch from bug#33870 has been already committed,
this bug#33992 is undeadlocked now.

The problem with xref-find-definitions is its unexpected outcome:
sometimes it pops up a window, sometimes doesn't.

It's like Russian roulette: pull the trigger, bang and there is
a hole in the screen that takes the form of glaringly empty space.

The proposed change is not to break the window configuration:
either split the selected window and show *xref* below,
or do inline placement, i.e. to replace the content of the
selected window with the *xref* buffer.