Ensure path is shortened correctly
This is to avoid a scenario described by @odeda:
Given project based dir ~/doc and a file at ~/documents/foo.txt (a different directory), wouldn't this code cause shortestPath() to return "uments/foo.txt"? This would not be useful.
Another underlying issue is that projectBaseDir property does not return the base directory of a document, but the currently active project. Because of this also https://bugs.kde.org/show_bug.cgi?id=459579 exists.