lib/pull: Don't fetch detached metadata twice for local pulls
lib/pull: Don't fetch detached metadata twice for local pulls

Obviously very minor, but I noticed this while working on `pull --reference`. If
we have a local repo, we'll have already done a hardlink and copied the detached
metadata too, so there's no reason to request it again via the fetcher path.

Closes: #978
Approved by: jlebon

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