Don't check ACLs in t/compare-trees on FreeBSD.

Treat FreeBSD like Cygwin and Darwin (OS X) in t/compare-trees.  This
should finish the process of disabling POSIX ACL support on FreeBSD
that was started here: 349ff15c7db09883abc20bdf3e8a4df6bff12cd3.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>