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check for valid upstream description string

bbockelm/cvmfs,Gangbiao/cvmfs,Moliholy/cvmfs,djw8605/cvmfs,cvmfs-testing/cvmfs,MicBrain/cvmfs,trshaffer/cvmfs,cvmfs/cvmfs,bbockelm/cvmfs,ruvolof/cvmfs,trshaffer/cvmfs,cvmfs-testing/cvmfs,MicBrain/cvmfs,trshaffer/cvmfs,MicBrain/cvmfs,djw8605/cvmfs,btovar/cvmfs,reneme/cvmfs,cvmfs/cvmfs,trshaffer/cvmfs,bbockelm/cvmfs,cvmfs/cvmfs,alhowaidi/cvmfsNDN,Gangbiao/cvmfs,reneme/cvmfs,DrDaveD/cvmfs,MicBrain/cvmfs,DrDaveD/cvmfs,bbockelm/cvmfs,cvmfs/cvmfs,DrDaveD/cvmfs,ruvolof/cvmfs,djw8605/cvmfs,DrDaveD/cvmfs,Gangbiao/cvmfs,ruvolof/cvmfs,reneme/cvmfs,alhowaidi/cvmfsNDN,cvmfs-testing/cvmfs,cvmfs/cvmfs,djw8605/cvmfs,ruvolof/cvmfs,cvmfs-testing/cvmfs,Moliholy/cvmfs,Gangbiao/cvmfs,alhowaidi/cvmfsNDN,btovar/cvmfs,DrDaveD/cvmfs,reneme/cvmfs,trshaffer/cvmfs,Moliholy/cvmfs,Gangbiao/cvmfs,btovar/cvmfs,cvmfs/cvmfs,bbockelm/cvmfs,alhowaidi/cvmfsNDN,btovar/cvmfs,Moliholy/cvmfs,btovar/cvmfs,alhowaidi/cvmfsNDN,cvmfs/cvmfs,Moliholy/cvmfs,MicBrain/cvmfs,DrDaveD/cvmfs,reneme/cvmfs,ruvolof/cvmfs,cvmfs-testing/cvmfs,djw8605/cvmfs,DrDaveD/cvmfs