Avoid branch detection for 'rpkg sources'
    
    After CentOS change 7a006813224aebdf3ce1d0d1667f97949af4c7a2, it
    was impossible (without explicit --release option) to download
    sources in unclean git branch.
    
    It would be pity to have such limitation everywhere, even for
    non-CentOS dist-git instances where branch info is not used for
    source location.  So let's make this feature optional:
    
        [cpkg]
        lookaside_kwargs = branch
    
    Also, the original motivation was that it a bit complicates 'rpkg'
    re-usability (rhbz#1410403).
    
    Signed-off-by: Pavel Raiskup <praiskup@redhat.com>