bug#20489: 25.0.50; next-error-find-buffer chooses non-current buffer without good reason

[Juri Linkov]

> It seems this discussion that came to a standstill here
> hopefully is revived and reached a preliminary consensus in bug#28864.
> Here's the current patch:

Dmitry, could you please confirm that this patch works with the current code
in xref--next-error-function?  If yes, I propose to push it to master.
Then we will get more feedback if something is broken :)

PS: When I typed the closing paren ‘)’ in the last sentence in ‘message-mode’,
it blinked on the letter ‘b’.  Do you know the reason of this weird behavior?