KABUL, Dec 26 (Reuters) - An Afghan former intelligence chief went on trial on Monday accused of war crimes and torture during communist rule in the 1980s, the first such trial to be held in Afghanistan after decades of warfare. Assadullah Sarwari has been detained since 1992, when Mujahideen (holy warrior) factions overthrew a Soviet-backed communist regime. Prosecutors said Sarwari was arrested for conspiring against Afghanistan's Islamic government and was guilty of illegal mass arrests and executions. Now 64 and sporting a short grey and black beard, he served as head of intelligence when thousands of people were killed for...