Add AppSettings to disable automatic "help" subcommand

[Xion]

When using SubCommands, clap will automatically include a "help" subcommand:

SUBCOMMANDS:
    help     Prints this message or the help of the given subcommand(s)

This is redundant, as subcommand help can also be obtained via $SUBCOMMAND --help.

Right now, it seems there is no way to hide that superfluous subcommand, as this doesn't work:

app.subcommand(SubCommand::with_name("help").setting(AppSettings::Hidden))

Consequently, the "help" subcommand may have to be handled explicitly.

Suggestion: introduce AppSettings::NoHelpSubcommand which, if set on the root App, hides the "help" subcommand from usage output.

[kbknapp]

Interesting, the fact that

app.subcommand(SubCommand::with_name("help").setting(AppSettings::Hidden))

Isn't hiding the subcommand looks like a bug. I'll dig into this further, because that's the intended behavior, and I would prefer that to an AppSettings::NoHelpSubcommand.

Also, small note, the help subcommand does a little more than the --help flag. It allows pulling the help from arbitrary other subcommands as well.

For instance, the following invocations are equivalent:

$ prog help push
$ prog push --help

$ prog help subcmd othercmd
$ prog subcmd othercmd --help

$ prog help
$ prog --help

[Xion]

For the record, I'm also using UnifiedHelpMessage. Not sure if it affects this.

What's also curious is that "help" subcommand is actually gone from the output of App::write_help, but it's there in the Err from App::get_matches_safe (so probably also in the stdout output of regular App::get_matches) called against empty argv, or argv with just help.

[kbknapp]

UnifiedHelpMessage shouldn't affect it at all. The part about being missing from write_help I'm assuming --help and --version are missing too. This is because unless a parsing method was calld (get_matches_*) those arguments and subcommands aren't built yet, since they're built lazily to allow users to override them (which is why this is a bug 😉)

[kbknapp]

I have this fixed in a local branch and will upload shortly. You can now either do as you were, Subcommand::with_name("help").setting(AppSettings::Hidden)) or use the new setting app.setting(AppSettings::DisableHelpSubcommand). The benefit of using the new setting is it actually disables the subcommand entirely, not just hides it. Also you can make it a global setting, and it will disable the help subcommand of all child subcommands as well.