Nikkei hits 2-wk high as energy firms shine; Showa Shell soars on M&A report

* Nikkei flat, Topix up 0.1 pct * Oil-related shares jump on merger news, rebounding oil prices * Trade thin as holiday period beckons * Nikkei may test new highs toward year-end - traders By Tomo Uetake TOKYO, Dec 22 (Reuters) - Japanese stocks briefly hit a two-week high on Monday morning, led by oil-related shares on rebounding crude prices and as Showa Shell Sekiyu surged over 25 percent on a media report of an impending takeover bid. The benchmark Nikkei share aver