SACRAMENTO, Calif., — The mayor of Sacramento, California, said on Thursday he was “horrified” by video showing police firing 20 times at an unarmed black man who died in the backyard shooting.Police in Sacramento, the state capital, released video footage late on Wednesday of the shooting of Stephon Clark, 22, on Sunday night, which police said showed him holding an object that was later found to be a cell phone.More than 200 demonstrators gathered on Thursday to denounce the shooting, in a protest organized by the Black Lives Matter movement.