[PATCH] add tests for --quiet option of diff programs

Signed-off-by: Alex Riesen <redacted>
---
This is almost the same test for exit code, it just also
counts the line on the output.

 t/t4017-quiet.sh |   79 ++++++++++++++++++++++++++
 1 files changed, 79 insertions(+), 0 deletions(-)
 create mode 100755 t/t4017-quiet.sh