Fix endless traversal loop
    
    Previously, the algorithm stopped traversing parent commits upon
    determining that the needn't be processed. This could leave commits
    where other, later merged branches were forked off in limbo: Some of
    their child commits were never traversed, so it could never be
    determined if they needed to be processed, thus they got put back to be
    revisitied, ad nauseam.
    
    The fix is to keep on traversing commits, but not process those whose
    results aren't needed.
    
    Fixes: #202
    
    Signed-off-by: Nils Philippsen <nils@redhat.com>