Update Commit Structure diagram with all fields in commit object

Previous generations of the Commit Structure diagram included all fields called out on the commit object. This update fixes the missing fields.

![screen shot 2013-09-30 at 1 47 51 pm](https://f.cloud.github.com/assets/352082/1240291/6169743a-2a09-11e3-9b65-945689226d32.png)