bug#8789: 23.3; debug backtrace buffer changes window on step-through

[Stefan Monnier]

>> Can you provide another patch that limits the effect to the
>> backtrace buffer?
> Can you?  I think that using `display-buffer-overriding-action' would be
> too harsh so we would have to specify this via the ACTION argument in
> (pop-to-buffer debugger-buffer).  A patch in that direction is below.

Looks fine to me, feel free to install,
thank you very much,


        Stefan