add helper `has_reference_log()` to `cvmfs_server`
add helper `has_reference_log()` to `cvmfs_server`

DrDaveD/cvmfs,Gangbiao/cvmfs,DrDaveD/cvmfs,reneme/cvmfs,Gangbiao/cvmfs,Gangbiao/cvmfs,reneme/cvmfs,trshaffer/cvmfs,reneme/cvmfs,djw8605/cvmfs,cvmfs/cvmfs,djw8605/cvmfs,DrDaveD/cvmfs,djw8605/cvmfs,cvmfs/cvmfs,cvmfs/cvmfs,alhowaidi/cvmfsNDN,DrDaveD/cvmfs,cvmfs/cvmfs,reneme/cvmfs,trshaffer/cvmfs,alhowaidi/cvmfsNDN,cvmfs/cvmfs,cvmfs/cvmfs,alhowaidi/cvmfsNDN,DrDaveD/cvmfs,alhowaidi/cvmfsNDN,DrDaveD/cvmfs,cvmfs/cvmfs,Gangbiao/cvmfs,reneme/cvmfs,Gangbiao/cvmfs,djw8605/cvmfs,trshaffer/cvmfs,trshaffer/cvmfs,trshaffer/cvmfs,djw8605/cvmfs,alhowaidi/cvmfsNDN,DrDaveD/cvmfs