Two issues since the filesystem migration;
- filepath.Base() doesn't do what the code expected when the path ends
in a slash, and we make sure the path ends in a slash.
- the return should be fully qualified, not just the last component.
With this change it works as before, and passes the new test for it.
GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4591
LGTM: imsodin
a4147d9019