Provide subcommand names for argument usage help

Before this commit args4j displayed only the metaVar and usage:

Unable to parse command line arguments: Argument "metaVar" is required
Valid options include:
 metaVar : usage

With this commit it displays all subcommand names:

Unable to parse command line arguments: Argument "metaVar" is required
Valid options include:
 [checkout | commit] : usage

This commit follows on to #76.