fs: use posixpath.relpath instead of self.relpath

Author: efiop

src/scmrepo/fs.py

@@ -160,7 +160,7 @@ def overlaps(cls, left, right):
     def relpath(self, path, start=None):
         if start is None:
             start = "."
-        return self.relpath(self.abspath(path), start=self.abspath(start))
+        return posixpath.relpath(self.abspath(path), start=self.abspath(start))
 
     def relparts(self, path, start=None):
         return self.parts(self.relpath(path, start=start))