fix: prepare hook is not executed for diff command (#425)

fixes https://github.com/roboll/helmfile/issues/424

note that the "cleanup" hook is already executed as it should

main.go

@@ -193,6 +193,9 @@ func main() {
 							return errs
 						}
 					}
+					if errs := state.PrepareRelease(helm, "diff"); errs != nil && len(errs) > 0 {
+						return errs
+					}
 
 					_, errs := executeDiffCommand(c, state, helm, c.Bool("detailed-exitcode"), c.Bool("suppress-secrets"))
 					return errs