upgrade: Refuse chronologically older commits unless --allow-downgrade

We don't want to allow MITM attackers to intercept upgrade requests
and provide clients with older OS versions vulnerable to security
flaws.

Only "ostree admin upgrade" gets this behavior for now - whether we
want to do it for "ostree admin switch" is another question.

src/libostree/ostree-core.c

@@ -1755,3 +1755,11 @@ ostree_commit_get_parent (GVariant  *commit_variant)
     return NULL;
   return ostree_checksum_from_bytes_v (bytes);
 }
+
+guint64
+ostree_commit_get_timestamp (GVariant  *commit_variant)
+{
+  guint64 ret;
+  g_variant_get_child (commit_variant, 5, "t", &ret);
+  return GUINT64_FROM_BE (ret);
+}