Use regular variables for git sources
    
    The current https://fedoraproject.org/wiki/Packaging:SourceURL spec
    suggests using e.g. `commit0` for the git commit variable.  The
    implication of this is that e.g. `Source0` uses `commit0`, `Source1`
    uses `commit1` etc.
    
    Move things this way so that the sources that come from git
    repositories (docker, docker-selinux, docker-storage-setup,
    docker-utils) are ordered first and consistently named.
    
    What I plan to do next is enhance
    https://github.com/cgwalters/rpmdistro-gitoverlay to understand how to
    fetch all component git repositories here.