On the surface, the documentary “A Life on the Farm” seems like an innocuous story of a loner farmer with a penchant for shooting his own home videos. That’s why filmmaker Oscar Harding was surprised that when his grandfather died, he left behind a videotape from longtime neighbor Charles Carson. Curious and confused, Harding gave the VHS a watch. What he saw was a home movie that felt a bit like a horror story, resulting in this disturbing and fascinating found footage documentary.