`bam diff` should exit with a return code

[hisplan]

It would be great if bam diff returns an exit code (0 for identical, 1 for different) just like the diff command does. This will allow us to do something like:

$ bam diff --in1 sample1.bam --in2 sample2.bam --all --out cmp
$ test $? -eq 0 && echo "Identical" || echo "Different"