Fifty years ago, Los Angeles was shocked by the horrific slaughter of actress Sharon Tate, then eight months pregnant, and four visitors to her Benedict Canyon estate. The next night, across town, two more people were murdered in their home: Leno and Rosemary LaBianca. At both houses, the victims were found stabbed to death, with variations of the word “pig” written in blood on the walls. When police identified the killers as a brainwashed “family” led by an ex-con named Charles Manson, the murders took on a mythical quality, becoming a source of perennial fascination. With the release of Quentin Tarantino’s “Once Upon a Time . . . in Hollywood” — which dramatizes the Manson Family’s crimes with a grisly, revisionist twist — the misconceptions about the historic case may only multiply.