* Fighting continues near Goma in eastern Congo
* Congo army says pushed back M23 rebel thrust
* U.N. brigade backed Congo army in clash -spokesman
By Jonny Hogg
GOMA, Democratic Republic of Congo, Aug 23 (Reuters) - A new U.N. combat brigade formed to try to neutralise armed groups in Congo by force if needed fired artillery at M23 rebels menacing the eastern border city of Goma anew, ...