Introduce FixedCompletions (#1574)

Example usage:

    choices := []string{"choice1", "choice2", "choice3"}
    cmd.RegisterFlagCompletionFunc(cobra.FixedCompletions(choices, ShellCompDirectiveNoFileComp))

completions.go

@@ -103,6 +103,14 @@ func NoFileCompletions(cmd *Command, args []string, toComplete string) ([]string
 	return nil, ShellCompDirectiveNoFileComp
 }
 
+// FixedCompletions can be used to create a completion function which always
+// returns the same results.
+func FixedCompletions(choices []string, directive ShellCompDirective) func(cmd *Command, args []string, toComplete string) ([]string, ShellCompDirective) {
+	return func(cmd *Command, args []string, toComplete string) ([]string, ShellCompDirective) {
+		return choices, directive
+	}
+}
+
 // RegisterFlagCompletionFunc should be called to register a function to provide completion for a flag.
 func (c *Command) RegisterFlagCompletionFunc(flagName string, f func(cmd *Command, args []string, toComplete string) ([]string, ShellCompDirective)) error {
 	flag := c.Flag(flagName)

completions_test.go

@@ -2664,3 +2664,30 @@ func TestCompleteWithRootAndLegacyArgs(t *testing.T) {
 		t.Errorf("expected: %q, got: %q", expected, output)
 	}
 }
+
+func TestFixedCompletions(t *testing.T) {
+	rootCmd := &Command{Use: "root", Args: NoArgs, Run: emptyRun}
+	choices := []string{"apple", "banana", "orange"}
+	childCmd := &Command{
+		Use:               "child",
+		ValidArgsFunction: FixedCompletions(choices, ShellCompDirectiveNoFileComp),
+		Run:               emptyRun,
+	}
+	rootCmd.AddCommand(childCmd)
+
+	output, err := executeCommand(rootCmd, ShellCompNoDescRequestCmd, "child", "a")
+	if err != nil {
+		t.Errorf("Unexpected error: %v", err)
+	}
+
+	expected := strings.Join([]string{
+		"apple",
+		"banana",
+		"orange",
+		":4",
+		"Completion ended with directive: ShellCompDirectiveNoFileComp", ""}, "\n")
+
+	if output != expected {
+		t.Errorf("expected: %q, got: %q", expected, output)
+	}
+}