ed2nav: optimisation in headsign handler
ed2nav: optimisation in headsign handler

the creation of the temporary vector was taking 12% of the time on
fr-idf.
Before this patch fr-idf was taking 4min30, after it took 3min50

patochectp/navitia,kadhikari/navitia,antoine-de/navitia,kadhikari/navitia,antoine-de/navitia,TeXitoi/navitia,ballouche/navitia,prhod/navitia,Tisseo/navitia,prhod/navitia,Tisseo/navitia,prhod/navitia,Tisseo/navitia,kinnou02/navitia,xlqian/navitia,patochectp/navitia,xlqian/navitia,kadhikari/navitia,pbougue/navitia,pbougue/navitia,TeXitoi/navitia,Tisseo/navitia,pbougue/navitia,ballouche/navitia,pbougue/navitia,lrocheWB/navitia,lrocheWB/navitia,ballouche/navitia,kinnou02/navitia,kadhikari/navitia,antoine-de/navitia,kinnou02/navitia,TeXitoi/navitia,prhod/navitia,Tisseo/navitia,CanalTP/navitia,lrocheWB/navitia,antoine-de/navitia,xlqian/navitia,xlqian/navitia,CanalTP/navitia,patochectp/navitia,patochectp/navitia,TeXitoi/navitia,CanalTP/navitia,CanalTP/navitia,kinnou02/navitia,ballouche/navitia,CanalTP/navitia,xlqian/navitia,lrocheWB/navitia