multi-pack-index: simplify handling of unknown --options

multi-pack-index: simplify handling of unknown --options

Although parse_options() can handle unknown --options just fine, none
of 'git multi-pack-index's subcommands rely on it, but do it on their
own: they invoke parse_options() with the PARSE_OPT_KEEP_UNKNOWN flag,
then check whether there are any unparsed arguments left, and print
usage and quit if necessary.

Drop that PARSE_OPT_KEEP_UNKNOWN flag to let parse_options() handle
unknown options instead, which has the additional benefit that it
prints not only the usage but an "error: unknown option `foo'" message
as well.

Do leave the unparsed arguments check to catch any unexpected
non-option arguments, though, e.g. 'git multi-pack-index write foo'.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>