Currently, if users try to use external diff command with arguments
will fail because the entire command won't be available through $PATH.
This patch allow users to use external diff tools with args (or not)
via KUBECTL_EXTERNAL_DIFF env.
Reference: https://github.com/kubernetes/kubectl/issues/937
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>
Kubernetes-commit: a6158c01b9add5b580059d24cb66e54c37ea2531