Fixes #8483 Windows completion error

This should fix #8483 I am not a Julia power user yet so could some of you guy's check it's working because I have not compiled base with it, @lucasb-eyer, @tkelman?
Actually, I'd say completely remove `\\` because on linux I can create a file or directory named `lol\hi`, I just tried it: `mkdir lol\\hi && ls`. Or is there any other repercussion I'm missing?