git p4: cygwin p4 client does not mark read-only

git p4: cygwin p4 client does not mark read-only

There are some old versions of p4, compiled for cygwin, that
treat read-only files differently.

Normally, a file that is not open is read-only, meaning that
"test -w" on the file is false.  This works on unix, and it works
on windows using the NT version of p4.  The cygwin version
of p4, though, changes the permissions, but does not set the
windows read-only attribute, so "test -w" returns false.

Notice this oddity and make the tests work, even on cygiwn.

Signed-off-by: Pete Wyckoff <pw@padd.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>