builtin/multi-pack-index.c: display usage on unrecognized command

builtin/multi-pack-index.c: display usage on unrecognized command

When given a sub-command that it doesn't understand, 'git
multi-pack-index' dies with the following message:

    $ git multi-pack-index bogus
    fatal: unrecognized subcommand: bogus

Instead of 'die()'-ing, we can display the usage text, which is much
more helpful:

    $ git.compile multi-pack-index bogus
    error: unrecognized subcommand: bogus
    usage: git multi-pack-index [<options>] write
       or: git multi-pack-index [<options>] verify
       or: git multi-pack-index [<options>] expire
       or: git multi-pack-index [<options>] repack [--batch-size=<size>]

        --object-dir <file>   object directory containing set of packfile and pack-index pairs
        --progress            force progress reporting

While we're at it, clean up some duplication between the "no sub-command"
and "unrecognized sub-command" conditionals.

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