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.
Also, I'm not sure where you're going with this, but from a completeness POV that callback is `stop`able. Maybe have a lower-level/private method that takes a block handle that, then use it for `fetchHeadEntriesWithError:` ?