`magit-display-process`: fix timed process buffer popup

Commit e26c9c7 moved the timed process buffer popup logic from `magit-run*`
into `magit-display-process`, but didn't take into account that it should
use `pop-to-buffer` instead of `display-buffer`.

When `magit-process-popup-time` was > 0 (as opposed to == 0), we *did*
use `display-buffer` instead of `pop-to-buffer`, but since its docstring
explicitly states "Popup the process buffer...", *always* use `pop-to-buffer`
when we have a running process and `magit-process-popup-time` is >= 0.

Fixes #987.

Reported-by: David Glasser <glasser@davidglasser.net>
Signed-off-by: Pieter Praet <pieter@praet.org>


@tarsius: I haven't caught up with `master` yet, so this branch is rooted
at fe063de9, but it merges just fine and shouldn't cause any problems.