pathutil: add doctests for canonpath() This is a followup to f445b10dc7fb. Since there's no way to ensure that more drive letters than C: exist, this seems like the only way to test it. This is enough to catch the f445b10dc7fb scenario, as well as CWD outside of the repo when the path isn't prefixed with path/to/repo.

share: handle --relative shares to a different drive letter gracefully This had the same problem as f445b10dc7fb. Banning os.path.relpath() is tempting, but the hint it provides is useful here.