bisect: add "git bisect help" subcommand to get a long usage string

bisect: add "git bisect help" subcommand to get a long usage string

Users are not often aware of the fact that "git bisect -h" can give
them a long usage description, as "git bisect" seems to accept only
dashless subcommands like "start", "good", ...

That's why this patch adds a "git bisect help" subcommand that just
calls "git bisect -h". This new subcommand is also fully documented
in the short usage string (that "git bisect" gives), in the long
usage string and in the man page (that "git help bisect" gives).

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