Implement StorageRoot for Git

[dbosk]

We have a StorageRoot implementation for AFS. But we want one for Git.

This should be done in two stages:

1. One StorageRoot for Git. This one can't do access control (as the AFS one can).
2. Another StorageRoot for GitHub (inherits from Git), which can also do the access control by adding access to the repo on GitHub. Maybe this will work the same for both global GitHub and self-hosted GitHub-enterprise.