bisect run: die if no command is given

It was possible to invoke "git bisect run" without any command.
This considers all commits as good commits since "$@"'s return
value for empty $@ is 0.

This is most probably not what a user wants (otherwise she would
invoke "git bisect run true"), so not providing a command now
results in an error.

Signed-off-by: Stephan Beyer <s-beyer@gmx.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-bisect.sh

@@ -450,6 +450,8 @@ bisect_replay () {
 bisect_run () {
 	bisect_next_check fail
 
+	test -n "$*" || die "$(gettext "bisect run failed: no command provided.")"
+
 	while true
 	do
 		command="$@"