bug#27879: Bad behavior with sr-speedbar package

[martin rudalics]

> 1. Start Emacs. It starts with a single window containing the *GNU Emacs* 
buffer.
>
> 2. Now toggle the sr-speedbar: M-x sr-speedbar-toggle. It contains two 
windows: on the right, the sr-speedbar; on the left the *GNU Emacs* buffer.
>
> 3. In *GNU Emacs* buffer visit a buffer: C-x C-f foo.txt. The *GNU Emacs* 
buffer window is divided (as in C-x 2) and foo.txt is in the bottom window.
>
> The last master with the right behavior is 2017-07-28 12:38:22 +0300, 
f1ed31a8f5c6f19aa5e119e670533241c6375945.

sr-speedbar advises ‘pop-to-buffer’.  It shouldn't do that or at least
do it in some different fashion.  The current behavior just breaks it.

martin