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

[Dmitry Gutov]

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.

In GNU Emacs 25.0.50.1 (x86_64-unknown-linux-gnu, GTK+ Version 3.12.2)
 of 2015-05-02 on axl
Windowing system distributor `The X.Org Foundation', version 11.0.11601901
System Description:     Ubuntu 14.10