Fixes bug in core.get_torrents_status where diff was always False

Fixes bug introduced in 5cd86aa5bcbd246f4ed6b4bed026cd72045304a3 where the diff
argument passed to torrentmanager.torrents_status_update is always False
instead of the value passed into get_torrents_status. This causes the sessionproxy
not to function properly which in turn increases CPU usage in the UIs.