Let "stg help" be like "stg --help"

Let "stg help" be like "stg --help"

"stg --help" prints the list of commands, but "stg help" just prints
the usage message for the help command. This may be useful in theory,
but the distinction is probably lost on 95% of all users (and the
remaining 5% probably don't need to see the usage message for the help
command anyway, since they know how it works). So just make both
commands output the helpful command list.

Signed-off-by: Karl Hasselström <kha@treskal.com>