Fix fetchHeadEntriesWithError:

Assert on initializing GTFetchHeadEntry in "GTFetchHeadEntriesCallback" when GTReference is nil following case.

Case:
1. Clone repository having 3 branches(master, branch1, branch2)
2. Checkout "branch2"
3. Fetch using "fetchRemote:withOptions:error:progress:"
4. Create array using "fetchHeadEntriesWithError:" -> assert

Can't create reference for "branch1"(no local branch) in this case.