Don't pass -AX to rsync in t/compare-trees if xattrs aren't supported.

Don't pass -AX to rsync in t/compare-trees if xattrs aren't supported.

The -AX options won't be available if rsync doesn't report the xattr
capability, so test for that before trying to use them.

Thanks to Yung-Chin Oei <yungchin@yungchin.nl> for the suggested
approach and initial report.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Reviewed-by: Zoran Zaric <zz@zoranzaric.de>