A test case introduced by 91479b9c (t7300: add tests to document
behavior of clean and nested git) uses 'chmod 0' to verify that a
subdirectory that has an unreadable .git file is not removed. This can
work only when the system pays attention to the permissions set with
'chmod'. Therefore, set the POSIXPERM prerequisite on the test case.
Signed-off-by: NJohannes Sixt <j6t@kdbg.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>