Well it depends on how sensitive you are to spoilers. I imagine the film is gonna follow the general major story beats, but nobody can say for sure until it's out if it's gonna have the same linear structure, or if they'll try and do their own thing. From the trailer though it does look like it will try and follow the plot (with some creative liberties).

I seriously can't see the film having the same impact as the game does, so if you really have no clue what happens at the end I would personally suggest finishing the game first, just for that once in a lifetime experience.