The Git connector doesn't specify which external nodes can be queryable. It is not realistic that Git content all be queryable (since Git repositories can be very large), so the connector should mark all but the following nodes as not queryable:
By having the master branch be queryable, JCR clients can search their latest codebase.
Note that people administering a particular ModeShape repository will still be able to configure a Git (or any) external source as never being queryable. This would override the connector's capabilities.