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.
I'm not 100% sure, but I guess no (as created by `GTRepository` they are not `nil`, I'm not sure if it will make sense for any to be `nil` once we have merging in place). How should we handle a nil there?