Fix exit status with -rq when there is a file in one directory but not another,
i.e., when print_only is called.

Prior to this change, -rq was always returning 0. After this change it will
return 1 if there is a difference between two directories.

This fixes compatibility with GNU diff and unbreaks backwards compatibility
expectations.

Found when trying to extend diff_test:brief_format_test.

MFC after: 2 months
MFC with: r321076, r321077