Update Source0 URL following the packaging guidelines
    
    Using Use https://fedoraproject.org/wiki/Packaging:SourceURL#Git_Tags
    allows the tarball to follow the common %{name}-%{version}.tar.gz
    pattern.
    
    Also simplify the .gitignore file to match all linkchecker-*.tar.gz
    files in the top-level directory.  This prevents fedpkg new-sources from
    adding a new line each time a new source is uploaded (which continue to
    grow because fedpkg does not remove the old files).