This lets you present a Git repository as a http.FileSystem to let
the http package take care of the HTTP for you.
This uses git2go which uses libgit2. This means you need a compatible version of libgit2 installed and accessible on your system. At some point libgit2 will have a static version, which will make this simpler.
The constructors let you do more or less work depending on how much information you have. You can pass in a tree, a reference or simply the name.
The tree will be used as the root of the filesystem.