builtin/gc.c: let parse-options parse 'git maintenance's subcommands

builtin/gc.c: let parse-options parse 'git maintenance's subcommands

'git maintenanze' 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.

This change makes 'git maintenance' consistent with other commands in
that the help text shown for '-h' goes to standard output, not error,
in the exit code and error message on unknown subcommand, and the
error message on missing subcommand.  There is a test checking these,
which is now updated accordingly.

Note that some of the functions implementing each subcommand don't
accept any parameters, so add the (unused) 'argc', '**argv' and
'*prefix' parameters 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>