Manage node body in git
If the content you are importing is not supposed to be edited, I would suggest to follow the same method used on Drupal.org for some nodes: Implement hook_node_view().
In the Drupal.org case, the hook is used to output dynamic content for nodes that once were written using the PHP filter.

This method would allow showing the updated content without to periodically import the content into the node body, it avoids creating revisions for the node, and it avoids having a node body that is edited, and made different from the Github content.
To improve the performance, you can cache the values grabbed from the github, and avoiding accessing it (through drupal_http_request()) too frequently.