bugreport: reject positional arguments

bugreport: reject positional arguments

git-bugreport already rejected unrecognized flag arguments, like
`--diaggnose`, but this doesn't help if the user's mistake was to forget
the `--` in front of the argument. This can result in a user's intended
argument not being parsed with no indication to the user that something
went wrong. Since git-bugreport presently doesn't take any positionals
at all, let's reject all positionals and give the user a usage hint.

Signed-off-by: Emily Shaffer <nasamuffin@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>