t0301: another fix for Windows compatibility

t0301: another fix for Windows compatibility

Just like 0fdcfa2f9f5 (t0301: fixes for windows compatibility,
2021-09-14) explained, we should not call `mkdir -m<mode>` in the test
suite because that would fail on Windows.

There was one forgotten instance of this which was hidden by a `SYMLINK`
prerequisite. Currently, this prevents this test case from being
executed on Windows, but with the upcoming support for symbolic links,
it would become a problem.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>