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.