LocalLister: always append trailing slash if it's a folder

Summary:
We were adding the trailing slash only when recursively calling scan(),
but we need to do the same also when calling Folder::append().

Test Plan:
1. filelight ~/home/foo/bar
2. go up to ~/home/foo
3. check whether /home/foo/bar has the trailing slash in the relevant map tooltips.

Differential Revision: https://phabricator.kde.org/D10801