* Heavy and small arms fire ring out before dawn
* Former rebels say city attacked by militia, former regime loyalists
* Fighting comes hours before U.N. vote on French mission (Adds further details of fighting)
By Emmanuel Braun and Paul-Marin Ngoupana
BANGUI, Dec 5 (Reuters) - Rival milita forces clashed in Central African Republic's capital on Wednesday hours before the U.N. was ...