fix: hide subcommand from short help when no subcommands

[olizilla]

ShortHelp always shows the subcommand help prompt even if there are no subcommands.

$ ipfs version
ipfs version 0.7.0

$ ipfs ls -h
USAGE
  ipfs ls <ipfs-path>... - List directory contents for Unix filesystem objects.

  ipfs ls [--headers | -v] [--resolve-type=false] [--size=false] [--stream | -s] [--] <ipfs-path>...

  Displays the contents of an IPFS or IPNS object(s) at the given path, with
  the following format:
  
    <link base58 hash> <link size in bytes> <link name>
  
  The JSON output contains type information.

  For more information about each command, use:
  'ipfs ls <subcmd> --help'

The last 2 lines of the short help are supposed to let the user know that --help will get them more help about this command. There should be no invitation to check subcommands if there are no subcommands for this command.

This PR changes things so we:

• only invite the user to check help for subcommands if there are any
• invite the user to check out the LongHelp for this command with --help

License: MIT
Signed-off-by: Oli Evans oli@tableflip.io

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[aschmahmann]

Left a few tiny suggestions/comments but LGTM. Thanks 😄.

Note: Does this effect our documentation generation at all or does it not utilize this codepath?

[aschmahmann]

Is it at all confusing that we let people know explicitly that they can run --help, but not the short -h?

I suspect not but figured I'd check in.

[aschmahmann]

Could you also add a test that checks on subcommands? The two things I'd like to watch for are:

1. I'd like some signal that these tests are still useful even if someone changes "For more information about each command" to be rephrased slightly. This could also be aided by checking a constant.
2. Making sure that subcommands are printed

[aschmahmann]

lowercase f here intentional?