Extra specs with uncommitted changes

@charlierudolph I added all missing specs for situations with uncommitted changes. Please take a look.
I would like the steps to be 

```
Given I have a feature branch named "feature"
And the following commits exist in my repository
  ...
And I am on the branch "feature"
And I have an uncommitted file with name: "uncommitted" and content: "stuff"
When I run `git extract refactor` with the last two commit shas
```

Similar for the other test.