Handle the situation where the branch asked is not found in the git repo
    
    This should fix an error seen in the logs:
      File "/usr/lib/python2.7/site-packages/pagure/internal/__init__.py", line 185, in mergeable_request_pull
        fork_obj.lookup_branch(request.branch_from).get_object().hex]
    AttributeError: 'NoneType' object has no attribute 'get_object'