Added new class and method to get entries of FETCH_HEAD after a fetch operation.

- Added new method in GTRepository to get the FETCH_HEAD entries.
- Added new class to represent entries in the FETCH_HEAD (GTFetchHeadEntry)

This will allow us to merge the fetched changed later on.
Yeah, I agree. Document that they can't be `nil` and then `NSParameterAssert` in the method.