builtin/show: Convert to direct return/decl-after-stmt style
builtin/show: Convert to direct return/decl-after-stmt style

I just happened to be reading this code; still testing
the waters for the new code style.  If we decide to do more
of this it's likely worth trying out a [Coccinelle](http://coccinelle.lip6.fr/)
patch or something.

Closes: #733
Approved by: jlebon

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree