TEST: add unit test for `History::Rollback()`
TEST: add unit test for `History::Rollback()`

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