cat-file: add missing [=<format>] to usage/synopsis

cat-file: add missing [=<format>] to usage/synopsis

When displaying cat-file usage, the fact that a <format> can
be specified is only visible when lookling at the --batch and
--batch-check options which are shown like this:

    --batch[=<format>]    show info and content of objects fed from the standard input
    --batch-check[=<format>]
                          show info about objects fed from the standard input

It seems more coherent and improves discovery to also show it
on the usage line.

In the documentation the DESCRIPTION tells us that "The output
format can be overridden using the optional <format> argument",
but we can't see the <format> argument in the SYNOPSIS above
the description which is confusing.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>