fix: Preserve ArgsLenAtDash during shell completion

[jonathan-conder]

Fixes #1877. This is just a workaround, a real fix is described in the issue itself.

Tested with:

func TestCompleteArgsLenAtDash(t *testing.T) {
	compareArgsLenAtDash(t, nil)
	compareArgsLenAtDash(t, []string{"foo"})
	compareArgsLenAtDash(t, []string{"--"})
	compareArgsLenAtDash(t, []string{"--", "foo"})
	compareArgsLenAtDash(t, []string{"--", "foo", "bar"})
	compareArgsLenAtDash(t, []string{"foo", "--"})
	compareArgsLenAtDash(t, []string{"foo", "--", "bar"})
	compareArgsLenAtDash(t, []string{"foo", "bar", "--"})
	compareArgsLenAtDash(t, []string{"--", "--"})
	compareArgsLenAtDash(t, []string{"--", "--", "foo"})
	compareArgsLenAtDash(t, []string{"--", "foo", "--"})
	compareArgsLenAtDash(t, []string{"foo", "--", "--"})
	compareArgsLenAtDash(t, []string{"foo", "--", "--", "bar"})
	compareArgsLenAtDash(t, []string{"foo", "--", "bar", "--"})
	compareArgsLenAtDash(t, []string{"foo", "bar", "--", "--"})
}

func compareArgsLenAtDash(t *testing.T, args []string) {
	argsLenAtDash1 := 0
	argsLenAtDash2 := 0

	cmd := &Command{
		Run: func(cmd *Command, args []string) {
			argsLenAtDash1 = cmd.ArgsLenAtDash()
		},
		ValidArgsFunction: func(cmd *Command, args []string, toComplete string) ([]Completion, ShellCompDirective) {
			argsLenAtDash2 = cmd.ArgsLenAtDash()
			return nil, ShellCompDirectiveDefault
		},
	}

	cmd.SetArgs(args)
	if err := cmd.Execute(); err != nil {
		t.Errorf("Unexpected error: %v", err)
	}

	cmd.SetArgs(append(append([]string{"__complete"}, args...), ""))
	if err := cmd.Execute(); err != nil {
		t.Errorf("Unexpected error: %v", err)
	}

	if argsLenAtDash1 != argsLenAtDash2 {
		t.Errorf("expected %v, got: %v", argsLenAtDash1, argsLenAtDash2)
	}
}

[marckhouzam]

@jonathan-conder I have seen much more activity on the pflag project recently. Would you be willing to implement the API proposed in #1877?