Fix subsetp based on patch by Benson and Steven Mitchell. Add appropriate short-circuits for null set arguments. Add tests (mostly from Steven Mitchell) to test suite.

#!/bin/sh
if
test -r $2
then
if
cmp $1 $2 > /dev/null
then
echo $2 is unchanged
rm -f $1
else
mv -f $1 $2
fi
else
mv -f $1 $2
fi