'infer subcommand' doesn't work with nested subcommands

[danieleades]

When InferSubcommands is enabled, subcommand inference only works for the top level of subcommands- to call nested subcommands the entire command must be written out.

use structopt::{StructOpt, clap::AppSettings};

#[derive(StructOpt)]
#[structopt(setting = AppSettings::InferSubcommands)]
enum Args {
    SomeCommand(SubSubCommand),
    AnotherCommand,
}

#[derive(StructOpt)]
#[structopt(setting = AppSettings::InferSubcommands)]
enum SubSubCommand {
    TestCommand,
}

fn main() {
    let args = Args::from_args();
}

• cargo run some-command test-command works
• cargo run some test-command works
• cargo run some-command test doesn't work
• cargo run some test doesn't work

i've tried both the above syntax, and explicitly marking with the #[structopt(subcommand] tag.

[CreepySkeleton]

This is the same problem as #151 , the same fix should apply