By Joe Cash and Xiuhao Chen BEIJING, Dec 31 (Reuters) - China's factory activity unexpectedly grew in December, snapping a record eight straight months of decline, lifted by a rise in pre-holiday orders as officials seek to spur the $19 trillion economy's manufacturing sector without worsening deflation. The official ​purchasing managers' index (PMI) rose to 50.1 in December from 49.2 in November, the National Bureau of Statistics' survey showed on Wednesday, topping the ‌50-point mark separating growth ...