Use dist-git repo and Rawhide in GNU Hello tutorial
    
    In practice, almost all packaging happens inside a dist-git repository.
    Creating new specfiles can be done outside of one,
    but it is also possible to use a dist-git compatible Git repository.
    
    In order to stay close to actual workflows,
    a dist-git-like repository is introduced to the packaging tutorial.
    
    Since 'rawhide' is the default branch for dist-git repositories,
    and normally all packaging work is first done in rawhide branch,
    the tutorial is also switched to use it.
    Also this moves the tutorial closer to actual workflows.
    The downside is that then the tutorial package is built for Rawhide,
    but installed in the user's system, which most probably is not Rawhide.
    However, due to GNU Hello's few dependencies,
    in practice this works well.
    There is also a way to avoid this problem,
    by introducing Toolbox and performing the test install there.