multi-pack-index: refactor "goto usage" pattern

multi-pack-index: refactor "goto usage" pattern

Refactor the "goto usage" pattern added in
cd57bc41bbc (builtin/multi-pack-index.c: display usage on unrecognized
command, 2021-03-30) and 88617d11f9d (multi-pack-index: fix potential
segfault without sub-command, 2021-07-19) to maintain the same
brevity, but in a form that doesn't run afoul of the recommendation in
CodingGuidelines about braces:

    When there are multiple arms to a conditional and some of them
    require braces, enclose even a single line block in braces for
    consistency[...]

Let's also change "argv == 0" to juts "!argv", per:

    Do not explicitly compare an integral value with constant 0 or
    '\0', or a pointer value with constant NULL[...]

I'm changing this because in a subsequent commit I'll make
builtin/commit-graph.c use the same pattern, having the two similarly
structured commands match aids readability.

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