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
I do not handle nothing because the regex should be garented a match due to the ´if´ at line 189.
About the ´nextind´
```julia
julia>s="α C:/"
julia>start = match(r" ",reverse(s)).offset
julia>s[length(s)-start]
'α'
```
So I need to step past the UTF-xx char. It could be changed to:
```julia
nextind(partial, length(partial) - m.offset) + 1
```
because I know the match char is ASCII.