git p4 test: use test_chmod for cygwin

git p4 test: use test_chmod for cygwin

This test does a commit that is a pure mode change, submits
it to p4 but causes the submit to fail.  It verifies that
the state in p4 as well as the client directory are both
unmodified after the failed submit.

On cygwin, "chmod +x" does nothing, so use the test_chmod
function to modify the index directly too.

Also on cygwin, the executable bit cannot be seen in the
filesystem, so avoid that part of the test.  The checks of
p4 state are still valid, though.

Thanks-to: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Pete Wyckoff <pw@padd.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>