zsh-completion: completion should always parse the root command!

It was running on the command it was invoked from which caused some
additional helpers (--help, --version) not to be generated.

zsh_completions.go

@@ -84,13 +84,15 @@ func (c *Command) GenZshCompletionFile(filename string) error {
 	return c.GenZshCompletion(outFile)
 }
 
-// GenZshCompletion generates a zsh completion file and writes to the passed writer.
+// GenZshCompletion generates a zsh completion file and writes to the passed
+// writer. The completion always run on the root command regardless of the
+// command it was called from.
 func (c *Command) GenZshCompletion(w io.Writer) error {
 	tmpl, err := template.New("Main").Funcs(funcMap).Parse(zshCompletionText)
 	if err != nil {
 		return fmt.Errorf("error creating zsh completion template: %v", err)
 	}
-	return tmpl.Execute(w, c)
+	return tmpl.Execute(w, c.Root())
 }
 
 func generateZshCompletionFuncName(c *Command) string {

zsh_completions_test.go

@@ -111,6 +111,23 @@ func TestGenZshCompletion(t *testing.T) {
 				`'\(\*-d \*--debug\)'{\\\*-d,\\\*--debug}`,
 			},
 		},
+		{
+			name: "command should run on the root command so --version and --help will be generated",
+			root: func() *Command {
+				r := &Command{
+					Use:     "mycmd",
+					Short:   "mycmd short description",
+					Version: "myversion",
+				}
+				s := genTestCommand("sub1", true)
+				r.AddCommand(s)
+				return s
+			}(),
+			expectedExpressions: []string{
+				"--version",
+				"--help",
+			},
+		},
 	}
 
 	for _, tc := range tcs {