fix: Fix fish shell completion bug that occurs when ValueEnum is the only one

[gw31415]

Bug to be fixed

This PR is about bug of fish shell completion.

Here is an example. The correct completion candidate is hoge, but \{hoge is actually completed.

$ example --action \{hoge       # extra left brace

This occurs when the ValueEnum has only one candidate.

Code to reproduce the bug

use std::io;

use clap::{CommandFactory, Parser, ValueEnum};
use clap_complete::{self, generate, Shell};

#[derive(Parser)]
#[command(name = "example")]
struct Args {
    #[arg(long)]
    action: Option<Action>,
}

#[derive(ValueEnum, Clone)]
enum Action {
    Hoge,
}

fn main() {
    Args::parse();
    let mut cmd = Args::command();
    let name = cmd.get_name().to_string();
    generate(Shell::Fish, &mut cmd, name, &mut io::stdout());
}

• The symptom

$ example --action \{hoge       # extra left brace

• Generated completion file

complete -c example -l action -r -f -a "{hoge	''}"
complete -c example -s h -l help -d 'Print help'

[epage]

Can you add a test case to https://github.com/clap-rs/clap/blob/master/clap_complete/tests/testsuite/fish.rs#L161 ?

Ideally

• First commit modifies the test, with the current buggy behavior
• Second commit fixes the code and updates the test with the desired behavior