Pass around more info between commit visitors
    
    Previously the visitors only passed around whether or not they needed
    further history for processing in the forward stage of processing git
    histories. This lets us pass around other information, e.g. if a
    changelog file is removed, so that its contents can be disregarded when
    processing the respective parent commit that introduced it.
    
    Related: #210
    
    Signed-off-by: Nils Philippsen <nils@redhat.com>