magit-process-setup: never collapse the section

Despite the face being used, too many users fail to realize that the
lines in the process buffer are sections and thus never try to show
the output by pressing TAB.

On the `next' branch this is being countered by creating all sections
expanded and only collapsing a section once the command returned
successfully, see 160ed937ff28bcf0c9fc84763376e7c88f3d836a.  That hasn't
made it to `master' yet, where we are doing something less reasonable.
So for now change `magit-process-setup' to never automatically collapse
a section.