Fix crash when trying to go to / on FM

Deals with more cases in FileChooser:compressPath, including `/mnt/..`.
Another cases which are now dealt with: `/..` becomes `/`, `/mnt//us` becomes `/mnt/us`.
Also, no need to do a `match`, since now we check the `pos` return of `gsub`.
Thanks for @felipensp for helping with the pattern and testing.