[WIP] Upgrade to LibGit2Sharp where that makes sense

Opened early for your pleasure and your feedback!
On my compturer **[updateNodeModules.cmd](https://github.com/projectkudu/kudu/blob/master/Kudu.Services.Web/updateNodeModules.cmd)** is triggered by the build of **[Kudu.Services.Web.csproj](https://github.com/projectkudu/kudu/blob/master/Kudu.Services.Web/Kudu.Services.Web.csproj#L277)**. This generates a `node_modules` under the Kudu.Services.Web hierarchy.

It's completely possible I misconfigured something. In which case, I'd very grateful for any guidance regarding this.