main: Treat default osname more consistently

The libostree already treats passing NULL for osname as "booted
osname, if any".  We should do the same inside the tools.  The upgrade
builtin had this logic duplicated there; we should be able to safely
remove it.

https://bugzilla.gnome.org/show_bug.cgi?id=710970

src/ostree/ot-admin-builtin-upgrade.c

@@ -70,8 +70,6 @@ ot_admin_builtin_upgrade (int argc, char **argv, OstreeSysroot *sysroot, GCancel
   if (!ot_admin_require_booted_deployment_or_osname (sysroot, opt_osname,
                                                      cancellable, error))
     goto out;
-  if (!opt_osname)
-    opt_osname = (char*)ostree_deployment_get_osname (ostree_sysroot_get_booted_deployment (sysroot));
   merge_deployment = ostree_sysroot_get_merge_deployment (sysroot, opt_osname); 
   if (merge_deployment == NULL)
     {

src/ostree/ot-admin-functions.c

@@ -67,6 +67,9 @@ ot_admin_complete_deploy_one (OstreeSysroot      *sysroot,
   deployments = ostree_sysroot_get_deployments (sysroot);
   booted_deployment = ostree_sysroot_get_booted_deployment (sysroot);
 
+  if (osname == NULL && booted_deployment)
+    osname = ostree_deployment_get_osname (booted_deployment);
+
   g_ptr_array_add (new_deployments, g_object_ref (new_deployment));
 
   for (i = 0; i < deployments->len; i++)