magit-display-process: use pop-to-buffer instead of display-buffer

Prior to commit e26c9c7 `magit-display-process' used `display-buffer'
and `magit-run*' used `pop-to-buffer' for timed display of the same
buffer.  The latter code was merged into `magit-display-process', which
resulted in `display-buffer' always being used.

This commit changes it to always use `pop-to-buffer'.

Fixes #987.