Implement --diff-args (#959)

Signed-off-by: Jan-Otto Kröpke <mail@jkroepke.de>

pkg/app/diff_test.go

@@ -16,6 +16,7 @@ import (
 
 type diffConfig struct {
 	args                   string
+	diffArgs               string
 	values                 []string
 	retainValuesFiles      bool
 	set                    []string
@@ -47,6 +48,10 @@ func (a diffConfig) Args() string {
 	return a.args
 }
 
+func (a diffConfig) DiffArgs() string {
+	return a.diffArgs
+}
+
 func (a diffConfig) Values() []string {
 	return a.values
 }