Redundant code in Zsh completion when there are no subcommands

[segevfiner]

Rust Version

rustc 1.23.0 (766bd11c8 2018-01-01)

Affected Version of clap

2.29.0

Expected Behavior Summary

There shouldn't be redundant code in the generated Zsh completion code.

Actual Behavior Summary

There is some redundant code when there are no sub-commands.

Generated from the sample:

#compdef clap-test

_clap-test() {
    typeset -A opt_args
    local ret=1

    local context curcontext="$curcontext" state line
    _arguments -s -S -C \
'--completions[]' \
'-h[Prints help information]' \
'--help[Prints help information]' \
'-V[Prints version information]' \
'--version[Prints version information]' \
&& ret=0
    
}

(( $+functions[_clap-test_commands] )) ||
_clap-test_commands() {
    local commands; commands=(
        
    )
    _describe -t commands 'clap-test commands' commands "$@"
}
(( $+functions[_clap-test_commands] )) ||
_clap-test_commands() {
    local commands; commands=(
        
    )
    _describe -t commands 'clap-test commands' commands "$@"
}

_clap-test "$@"

Notice the double _clap-test_commands part.

Steps to Reproduce the issue

1. Build the sample.
2. Run cargo run -- --completions.
3. Check the output.

Sample Code or Link to Sample Code

extern crate clap;

use std::io;
use clap::{App, Arg, Shell};

fn build_cli() -> App<'static, 'static> {
    App::new("clap-test")
        .arg(Arg::with_name("completions")
            .long("completions"))
}

fn main() {
    let matches = build_cli().get_matches();

    if matches.is_present("completions") {
        build_cli().gen_completions_to("clap-test", Shell::Zsh, &mut io::stdout())
    }
}

Debug output

clap-debug.txt

[kbknapp]

@segevfiner since you're already on a roll for these completions PRs, do you want to take a swing at this one? 😄

[segevfiner]

I probaby will 😉