test-cat-file.sh: use -l instead of -b; explain differences.

It turns out that cmp -b doesn't work on at least NetBSD, so use -l
instead, and when there's a problem, diff the "bup meta" output as
well.

Thanks to Thomas Klausner <tk@giga.or.at> for the report.

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