show: Fix segfault if we can't find an object

We need to use the original rev here.
core: Add OSTREE_COMMIT_GVARIANT_STRING

For concatenation, we need one without the G_VARIANT_TYPE().
delta: Move to-commit into superblock

Also, add the from/to checksums there as well.  The idea here is that
if we want to do any kind of version comparisons, we can just get that
from the superblock directly, rather than doing another fetch for the
to-commit.