resolves #193: simpler commit tables

@kevgo 

Can now remove the duplication from all commit tables. I removed them from just git-extract for now. Will remove it from the others, but wanted to give you a chance to look at it now.
Class-level documentation is important, so I'd rather keep this verification in. Let's tune this comment a bit: 

```ruby
# Monkey patches to the Hash class that make testing easier
```