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.

Can you indent this with tabs ?

Also, since it means the enumeration can fail, I think the enumeration block's signature should be changed to `(GTFetchHeadEntry *entry, NSError *error, BOOL *stop)` so the error can be seen by the client instead of silently swallowing it ?