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 agree, but then the callback function for `git_repository_fetchhead_foreach` has this parameters: `(const char *ref_name, const char *remote_url, const git_oid *oid, unsigned int is_merge, void *payload)`. I modelled the order after it (made sense to me). Thoughts?