Currently, diff AND kubectl can return 1 errors either when diff finds
differences, or when kubectl fails something. It also prints an ugly
error when diff finds a differences, since 1 is treated as an error.
Two things this PR does:
1. If diff returns 1, then do not treat it as an error, and exit with
exit code 1. It no longer prints the ugly error.
2. Kubectl errors are +1'd so that they never return 1 which shouldn't
be considered an error.
We need to update the documentation accordingly, to mention that
`KUBECTL_EXTERNAL_DIFF` programs must also follow the convention of
using one as their exit code for changes detected.
Kubernetes-commit: f2b21f08d95291212ba8987aa9d446c02a96089a
4c36396b35