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.
It's weird, but all files are duplicated. Apparently, they are once for `ObjectiveGit` framework target, and once for `ObjectiveGit-iOS`.