Multi-commands (such as rbt review) had an issue where usage and help

output would be displayed multiple times. On Python 2.7, we'd get a

usage + help block, followed by a usage block, followed by another

usage + help block. On newer versions, we'd just get two usage + help

blocks.
The reason this was happening has to do with how we handle help in

RBTools. We have our own custom --help handler that's invoked before

any commands are set up, which allows us to do rbt --help,

rbt <command> --help, or rbt --help <command>, along with supporting

passing --help to any third-party rbt-* scripts.
When handling help for internal commands, we locate the right command

class, check if it's a multi-command or a standard command, and attempt

to get the right help information. For multi-commands, we do an explicit

parse of the options, which will funnel the --help to the right

subcommand if needed.
This is where the problem was found. This test parse will see the

--help and will result in ArgumentParser.print_help() being called,

resulting in one usage + help block.
Then, on Python 2.7, the argument parser notes that we haven't passed

any required arguments for the subparser and triggers

ArgumentParser.error(), which prints a second usage block.
We then return formatted help, which includes the final usage + help

block. This gets printed to the console.
Solving this requires babysitting argparse a bit more.
We no longer leave invocation to chance. Instead, we first check if

there's a multi-command class, and then grab the first positional

argument. If it matches a known subcommand, we pull out the class and

build a new, fixed set of command line arguments that will generate new

help output.
This gives us full control of how help output is generated. We no longer

need to stub argparse when invoking --help on a subcommand, and

we're far less sensitive to Python version differences or the position

of help or -h/--help.

Tested the following on Python 2.7 and 3.8:

rbt help
rbt help <command>
rbt help <multi-command>
rbt help <multi-command> <subcommand>
rbt --help
rbt --help <command>
rbt --help <multi-command>
rbt --help <multi-command> <subcommand>
rbt <command> --help
rbt <multi-command> --help
rbt <multi-command> --help <subcommand>
rbt <multi-command> <subcommand> --help