Some cleanup.

* bzrlib/tests/blackbox/test_update.py:
(TestUpdate.test_update_checkout_prevent_double_merge): Use full
lines for changes to get a cleanly formatted conflicted file
content.
(TestUpdate.test_update_remove_commit): Add graph variable
definitions from review.

* bzrlib/tests/blackbox/test_commit.py:
(TestCommit.test_commit_a_text_merge_in_a_checkout): Use full
lines for changes to get a cleanly formatted conflicted file
content.

* bzrlib/workingtree.py:
(WorkingTree._update_tree): Use 'nb_conflicts' instead of 'result'
to clarify.