pull: Fix theoretical checksum collision for metadata fetches
pull: Fix theoretical checksum collision for metadata fetches

I was making some other changes in this code, and noticed that we were adding
checksums without object types into the same hash table for metadata. We should
*never* do this with both metadata content objects, since in theory a content
object could have the same hash as metadata.

I don't actually think it's possible in practice for pure metadata to collide,
since they have different structures, but let's do this anyways since it's
conceptually right.

Closes: #651
Approved by: giuseppe

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