Support usage as plugin for tools like kubectl

In this case the executable is `kubectl-plugin`, but we run it as:

    kubectl plugin

And the help text should reflect the actual usage of the command.

To create a plugin create the root command like:

    rootCmd := &cobra.Command{
        CommandName: "kubectl plugin",
    }

When `CommandName` is set, Name() use it as is instead of guessing the
command name from the `Use` line.

Issues:
- Need to update the docs.

Fixes: #2017
Signed-off-by: Nir Soffer <nsoffer@redhat.com>

command.go

@@ -57,6 +57,10 @@ type Command struct {
 	// Example: add [-F file | -D dir]... [-f format] profile
 	Use string
 
+	// CommandName is the command name if set. Otherwise the command
+	// name is the first word of the `Use` line.
+	CommandName string
+
 	// Aliases is an array of aliases that can be used instead of the first word in Use.
 	Aliases []string
 
@@ -1441,8 +1445,11 @@ func (c *Command) DebugFlags() {
 	debugflags(c)
 }
 
-// Name returns the command's name: the first word in the use line.
+// Name returns CommandName or the first word in the use line.
 func (c *Command) Name() string {
+	if c.CommandName != "" {
+		return c.CommandName
+	}
 	name := c.Use
 	i := strings.Index(name, " ")
 	if i >= 0 {

command_test.go

@@ -366,6 +366,29 @@ func TestAliasPrefixMatching(t *testing.T) {
 	EnablePrefixMatching = defaultPrefixMatching
 }
 
+// TestPlugin checks usage as plugin for aother commmand such as kubectl.  The
+// executable is `kubectl-plugin`, but we run it as `kubectl plugin`. The help
+// text should reflect the way we run the commmand.
+func TestPlugin(t *testing.T) {
+	rootCmd := &Command{CommandName: "kubectl plugin", Args: NoArgs}
+	subCmd := &Command{Use: "sub [flags]", Args: NoArgs, Run: emptyRun}
+	rootCmd.AddCommand(subCmd)
+
+	rootHelp, err := executeCommand(rootCmd, "-h")
+	if err != nil {
+		t.Errorf("Unexpected error: %v", err)
+	}
+
+	checkStringContains(t, rootHelp, "kubectl plugin [command]")
+
+	childHelp, err := executeCommand(rootCmd, "sub", "-h")
+	if err != nil {
+		t.Errorf("Unexpected error: %v", err)
+	}
+
+	checkStringContains(t, childHelp, "kubectl plugin sub [flags]")
+}
+
 // TestChildSameName checks the correct behaviour of cobra in cases,
 // when an application with name "foo" and with subcommand "foo"
 // is executed with args "foo foo".