Work on the logic to generate the changelog
    
    In this commit, we are dropping the dependency on pygit2.
    Fixes https://pagure.io/Fedora-Infra/rpmautospec/issue/10
    
    We also take into account the presence and content of the
    `changelog` file in the git repo as the idea is only to
    automatically generate the changelog up until the latest commit
    updating this `changelog` file.
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>