[MERGE] Convert tests.blackbox.test_b* to use internals where appropriate

[Martin Pool]

Martin Pool has voted approve.
Status is now: Approved
Comment:

That looks really nice, thanks.

+        base_tree = branch.bzrdir.open_workingtree()
+        base_tree.lock_write()
+        base_tree.add(['a', 'b'])
+        base_tree.commit('init')
+        base_tree.unlock()

As a separate patch we could factor out a test method that makes a 
branch and tree and makes one or two commits in it.  It would need some 
care to cover the common cases while still being simpler than just 
calling the branch methods directly.

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3C46B327B2.2020305%40warwick.ac.uk%3E