make dev.clone.ssh errors when using repository forks

[rgraber]

Running make dev.clone.ssh results in fatal: ref refs/remotes/origin/HEAD is not a symbolic ref if you are working on a fork for any one of the devstack services. This is particularly problematic for repositories like course-discovery, where most people don't have write access and thus have to work on forks for development. This also hinders running update-dbs-init-sql-scripts.sh because that calls make dev.clone.ssh