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?
This seems like the wrong place to fix this – a dangling backslash in a shell-quoted expression is still an error. In `C:\\` the backslash is already "quoted" by the first backslash. I may be missing something here though.