Fix --version help with CommandDisplayNameAnnotation

When setting Command.Version, a --version option is added. The help
message for the --version command did not consider the command display
name:

    Flags:
      -h, --help      help for kubectl plugin
      -v, --version   version for kubectl-plugin

With this change the help test is consistent with other flags:

    Flags:
      -h, --help      help for kubectl plugin
      -v, --version   version for kubectl plugin

command.go

@@ -1216,7 +1216,7 @@ func (c *Command) InitDefaultVersionFlag() {
 		if c.Name() == "" {
 			usage += "this command"
 		} else {
-			usage += c.Name()
+			usage += c.displayName()
 		}
 		if c.Flags().ShorthandLookup("v") == nil {
 			c.Flags().BoolP("version", "v", false, usage)

command_test.go

@@ -371,8 +371,9 @@ func TestAliasPrefixMatching(t *testing.T) {
 // text should reflect the way we run the command.
 func TestPlugin(t *testing.T) {
 	cmd := &Command{
-		Use:  "kubectl-plugin",
-		Args: NoArgs,
+		Use:     "kubectl-plugin",
+		Version: "1.0.0",
+		Args:    NoArgs,
 		Annotations: map[string]string{
 			CommandDisplayNameAnnotation: "kubectl plugin",
 		},
@@ -386,13 +387,15 @@ func TestPlugin(t *testing.T) {
 
 	checkStringContains(t, cmdHelp, "kubectl plugin [flags]")
 	checkStringContains(t, cmdHelp, "help for kubectl plugin")
+	checkStringContains(t, cmdHelp, "version for kubectl plugin")
 }
 
 // TestPlugin checks usage as plugin with sub commands.
 func TestPluginWithSubCommands(t *testing.T) {
 	rootCmd := &Command{
-		Use:  "kubectl-plugin",
-		Args: NoArgs,
+		Use:     "kubectl-plugin",
+		Version: "1.0.0",
+		Args:    NoArgs,
 		Annotations: map[string]string{
 			CommandDisplayNameAnnotation: "kubectl plugin",
 		},
@@ -408,6 +411,7 @@ func TestPluginWithSubCommands(t *testing.T) {
 
 	checkStringContains(t, rootHelp, "kubectl plugin [command]")
 	checkStringContains(t, rootHelp, "help for kubectl plugin")
+	checkStringContains(t, rootHelp, "version for kubectl plugin")
 	checkStringContains(t, rootHelp, "kubectl plugin [command] --help")
 
 	childHelp, err := executeCommand(rootCmd, "sub", "-h")