bisect: add first-parent option to documentation

Ensure that the [--first-parent] option is listed in the output of
"git bisect -h".

Signed-off-by: Aaron Lipman <alipman88@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-bisect.sh

@@ -4,7 +4,7 @@ USAGE='[help|start|bad|good|new|old|terms|skip|next|reset|visualize|view|replay|
 LONG_USAGE='git bisect help
 	print this long help message.
 git bisect start [--term-{old,good}=<term> --term-{new,bad}=<term>]
-		 [--no-checkout] [<bad> [<good>...]] [--] [<pathspec>...]
+		 [--no-checkout] [--first-parent] [<bad> [<good>...]] [--] [<pathspec>...]
 	reset bisect state and start bisection.
 git bisect (bad|new) [<rev>]
 	mark <rev> a known-bad revision/