gdm: mute debug messages

Since we're just using a cut-and-paste of gdm code there's a
slight impedance mismatch between how that code and the rest of
the shell manage debug messages.  In GDM, they're out of sight by
default, but in gnome-shell they're visible and quite verbose.
This muddies up and masks the useful messages that other parts
of the code generate.

This commit just mutes the messages unconditionally by default,
since they aren't that useful anyway.

src/gdmuser/gdm-user-manager.c

@@ -3057,6 +3057,15 @@ gdm_user_manager_finalize (GObject *object)
         G_OBJECT_CLASS (gdm_user_manager_parent_class)->finalize (object);
 }
 
+static void
+gdm_user_muted_debug_log_handler (const char     *log_domain,
+                                  GLogLevelFlags  log_level,
+                                  const char     *message,
+                                  gpointer        data)
+{
+        /* Intentionally empty to discard message */
+}
+
 /**
  * gdm_user_manager_ref_default:
  *
@@ -3075,6 +3084,11 @@ gdm_user_manager_ref_default (void)
                 user_manager_object = g_object_new (GDM_TYPE_USER_MANAGER, NULL);
                 g_object_add_weak_pointer (user_manager_object,
                                            (gpointer *) &user_manager_object);
+
+                /* We don't normally care about user manager messages in the shell,
+                 * so mute them */
+                g_log_set_handler (G_LOG_DOMAIN, G_LOG_LEVEL_DEBUG,
+                                   gdm_user_muted_debug_log_handler, NULL);
         }
 
         return GDM_USER_MANAGER (user_manager_object);