use worktree's rather than cloning whole repository
    
    Before:
    16.69user 5.00system 0:21.16elapsed 102%CPU (0avgtext+0avgdata 240120maxresident)k
    0inputs+59544outputs (0major+1461900minor)pagefaults 0swaps
    
    After:
    14.16user 4.82system 0:19.04elapsed 99%CPU (0avgtext+0avgdata 42576maxresident)k
    8inputs+8outputs (0major+1369338minor)pagefaults 0swaps
    
    Signed-off-by: Igor Gnatenko <ignatenkobrain@fedoraproject.org>