deploy: Return staged deployment

Today rpm-ostree has some code to run a "sanitycheck" on a deployment.
I had initially deleted that when adapting it to use the staging code,
but I realized it should work fine; we just won't see the merged
config, but that's OK.

When I readded that code it started crashing because we didn't
actually return the new deployment object.  We'll gain some coverage
here as I'll land the code to have rpm-ostree use staging, then bump
the rpm-ostree tests here.

Closes: #1559
Approved by: jlebon

src/libostree/ostree-sysroot-deploy.c

@@ -2736,6 +2736,7 @@ ostree_sysroot_stage_tree (OstreeSysroot     *self,
   if (!ostree_sysroot_load (self, cancellable, error))
     return FALSE;
 
+  ot_transfer_out_value (out_new_deployment, &deployment);
   return TRUE;
 }