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

[Juri Linkov]

>> So the question is: in xref--next-error-function can we use code similar
>> to what is used in compilation-next-error-function that works without 
>> problems?
>
> You probably mean compilation-goto-locus (it contains the navigation part),
> but even so, it's not easy to tell which part you mean (the function does
> more than one would expect).
>
> In short, adapting that code is kind of difficult, but hopefully I found
> and fixed the problem in xref directly
> in 11c58c4fc495ea4f7bff52ca077fd3e4382aa900.

While looking at this, I discovered more problems:

1. the same problem that you fixed for xref, also exists in occur:
   it doesn't set the right current-buffer for next-error.

2. also typing RET in *Occur* doesn't set next-error-last-buffer.
   Maybe we need to set it in occur-mode-goto-occurrence (but not in 
occur-next-error)

3. typing RET in *xref* should set next-error-last-buffer as well

4. Maybe these 2 lines needed to be added also to xref--xref-buffer-mode:

        (setq next-error-last-buffer buffer)
        (setq-default next-error-last-buffer buffer)

like in next-error to set both buffer-local and global values.