Fix path completion when UTF8 char is occurring before match characters.

Small fix to #8838 so completion works again on: `"β $dir_space\\space"`, @tkelman, @ivarne
@stevengj I have found the solution, it converts to charecter index and the back:
```
julia> s="αβ C:/αβ";
julia> start = chr2ind(s, length(s) - ind2chr(s, match(r" ",reverse(s)).offset) + 2);
julia> s[start:end]
"C:/αβ"
```