spec: fix Requires/BuildRequires on git
    
    For a long time we use 'dirxz' as ARCHIVE_FORMAT (we distribute
    archive.dir.tar.xz) in fedora package.  That's been this way
    because 'xz' is historically installed by default in minimal
    buildroot and because that's the preferred distribution method
    (smallest size of the archive file).
    
    Because 'dirxz' is preferred anyway, remove the redundant
    --disable-git %configure option.  But also to avoid surprises in
    future, let's make the 'dirxz' explicit by having 'BuildRequires:
    xz' and using '--enable-xz' configure option.
    
    As it was just a redundant dependency, from now on we don't depend
    on 'git'.
    
    Related: rhbz#1161284, rhbz#1416691
    Version: 0.19.8.1-7