You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: main_test.go
    path of file: ./repos/gitkube/vendor/golang.org/x/tools/cmd/bundle
    
    the code of the file until where you have to start completion: // Copyright 2015 The Go Authors. All rights reserved.
// Use of this sour
    
func diff(a, b string) string {
	var cmd *exec.Cmd
	switch runtime.GOOS {
	case "plan9":
		cmd = exec.Command("/bin/diff", "-c", a, b)
	default:
		cmd = exec.Command("/usr/bin/diff", "-u", a, b)
	}
	var out bytes.Buffer
	cmd.Stdout = &out
	cmd.Stderr = &out
	cmd.Run() // nonzero exit is expected
	if out.Len() == 0 {
		return "(failed to compute diff)"
	}
	return out.String()
}