Fix help output for nested subcommands

Only 'ostree admin' was appearing in the help message for commands
nested within ostree admin.

https://bugzilla.gnome.org/show_bug.cgi?id=737194

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

@@ -151,6 +151,8 @@ ot_admin_builtin_instutil (int argc, char **argv, OstreeSysroot *sysroot, GCance
       goto out;
     }
 
+  g_set_prgname (g_strdup_printf ("ostree admin instutil %s", subcommand_name));
+
   if (!subcommand->fn (argc, argv, sysroot, cancellable, error))
     goto out;
  

src/ostree/ot-builtin-admin.c

@@ -179,6 +179,8 @@ ostree_builtin_admin (int argc, char **argv, OstreeRepo *repo, GCancellable *can
       goto out;
     }
 
+  g_set_prgname (g_strdup_printf ("ostree admin %s", subcommand_name));
+
   sysroot_path = g_file_new_for_path (opt_sysroot);
   sysroot = ostree_sysroot_new (sysroot_path);
   if (!subcommand->fn (argc, argv, sysroot, cancellable, error))