builtin/sparse-checkout.c: let parse-options parse subcommands

builtin/sparse-checkout.c: let parse-options parse subcommands

'git sparse-checkout' parses its subcommands with a couple of 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 some of 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>