cargo bench fails with multiple --bench or --test arguments

[rwakulszowa]

cargo bench runs all benchmarks
cargo bench --bench=a runs benchmark a
cargo bench --bench=a --bench=b gives error: Invalid arguments.
cargo bench --test=a --test=b gives error: Invalid arguments.

cargo test --test=a --test=b runs test suites a and b
cargo test --bench=a --bench=b runs test suites for benches a and b

It seems that cargo bench handles its command line arguments differently than cargo test.

[alexcrichton]

Thanks for the report! Probably a small bug with how we parse the command line arguments here...