bisect run: die if no command is given

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 <redacted>
Signed-off-by: Junio C Hamano <redacted>