Fix bug #394227, osutils.relpath() could get into an infinite loop.
If your current path was at the root of a drive (like 'C:/', and you did something like 'bzr init-repo H:/repo', then the relpath code would spin forever, trying to find the relative path to H:/repo from C:/. The problem was that os.path.split() doesn't strip off the drive letter, so you end up comparing C:/ to H:/, and neither is 'shorter' to say that we didn't match.