When diff(1) finds differences, it returns an exit status of 1.

When diff(1) finds differences, it returns an exit status of 1.
In that particular case, refrain from printing "diff: exit code 1"
because that exit status doesn't indicate an error condition.

Issue noticed by and patch OK'ed by ajacoutot@.
"I agree with the goal, and I suspect the diff actually achieves it" guenther@.