Simplify gdfo computing by finding tails when at graph build time.

* bzrlib/_known_graph_pyx.pyx:
(KnownGraph._get_or_create_node): We need to know if the ndoe as
created.
(KnownGraph._initialize_nodes): Calulate tails ahead of time to
intialize gdfo computing.
(KnownGraph._find_gdfo): Use tails directly.

* bzrlib/_known_graph_py.py:
(KnownGraph._initialize_nodes): Calulate tails ahead of time to
intialize gdfo computing.
(KnownGraph._find_gdfo): Use tails directly.