builtin/multi-pack-index.c: let parse-options parse subcommands

builtin/multi-pack-index.c: let parse-options parse subcommands

'git multi-pack-index' parses its subcommands with a couple of if-else
if statements.  parse-options has just learned to parse subcommands,
so let's use that facility instead, with the benefits of shorter code,
handling missing or unknown subcommands, and listing subcommands for
Bash completion.

Note that the functions implementing each subcommand only accept the
'argc' and '**argv' parameters, so add a (unused) '*prefix' parameter
to make them match the type expected by parse-options, and thus avoid
casting function pointers.

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