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?
If the string ends on `\\` it give a error even though `cd C:\\` is valid on windows:
```julia
julia> Base.shell_parse("cd C:\\",true)
ERROR: dangling backslash
 in shell_parse at string.jl:1146
```
And if string ends on `cd C:\\P` is parsed then it splits the dir path and this makes the `complete_path` complete on only `"P"` instead of the path.
```Julia
julia> Base.shell_parse("cd C:\\P",true)
(:((("cd",),("C:","P"))),0:-1)
```