use ensure_packages to install git-core

I was running into package conflicts over git-core, which can be remedied by using ensure_packages.  I reduced the git-core package declaration into a set of dependencies, then moved git-core into the $system_packages array.

By applying this patch, it becomes possible to include other puppet modules that specify git-core using ensure_packages.

It's my belief that this patch introduces no net change to the puppet dependency graph compared to the way the git-core package is currently specified in master, but I am not a super puppet hacker and I could be wrong. I am curious to hear any feedback about the patch.