bundle <cmd>: have usage_msg_opt() note the missing "<file>"

bundle <cmd>: have usage_msg_opt() note the missing "<file>"

Improve the usage we emit on e.g. "git bundle create" to note why
we're showing the usage, it's because the "<file>" argument is
missing.

We know that'll be the case for all parse_options_cmd_bundle() users,
as they're passing the "char **bundle_file" parameter, which as the
context shows we're expected to populate.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>