bug#20493: 25.0.50; next-error in change-log-mode can hide its buffer

[Juri Linkov]

> next-error-function in *compile* or *grep* works in a certain way:
>
> - It doesn't open the locations when the *compile* buffer is selected,
>   it just jumps between them in that buffer.
>
> - When *compile* is visible but displayed a different window, pressing
>   C-x ` will never display the "next error" on top of *compile*. So the
>   latter always stays visible.
>
> Neither is true of change-log-next-error.
>
> I think we need more consistency in this.

This is finally implemented.