The readme has a broken link to openrover-demo.repos. I was unable to find the file in the package history.
In the meantime, here are the repos and branches I had to clone to build on eloquent:
github.com/BehaviorTree/BehaviorTree.CPP ros2-devel
github.com/RoverRobotics/bno055_driver master
github.com/RoverRobotics/keystroke-ros2 master
github.com/ros-planning/navigation2 eloquent-devel
github.com/allenh1/rplidar_ros ros2
github.com/RoverRobotics-forks/serial-ros2 master
Also, in openrover_demo/package.xml I had to remove the nav2_world_model dependency, as it is no longer in navigation2.