Been trying for 30 years to figure out the name of this movie...?

There was a movie my sister and I watched during a late night summer stay-up in the 1970's. We never knew the name. It was about two girls, one who was abducted and murdered and her friend (I think in the French countryside) tried to escape who she THOUGHT was the killer, a man who appeared suspicious and she ran around a large tree trunk then whacked him with a piece of wood thinking she killed him. In reality the killer was the giant policeman who grabbed her as she ran to him for safety. The shot of of her face as she realized HE was the real killer and the guy she whacked was trying to save her was tremendously effective. The guy she whacked came around the other side of the tree and kills the cop and saves her. We never knew the title only the movie and I cannot remember if it was French with English subtitles. Anyone know of this movie?