'bzr switch' opens too many branches too many times

I just wrote an effort test for 'bzr switch --create-branch' to make sure we didn't open master branch multiple times.

It turns out that we open things *way* too much. Without my fix, we open the checkout branch 10 times, master branch 2 times, and the target branch 3 times.

    def test_switch_create_doesnt_open_master_branch(self):
        master = self.make_branch_and_tree('master')
        master.commit('one')
        # Note: not a lightweight checkout
        checkout = master.branch.create_checkout('checkout')
        opened = []
        def open_hook(branch):
            # Just append the final directory of the branch
            opened.append(branch.base.rstrip('/').rsplit('/', 1)[1])
        branch.Branch.hooks.install_named_hook('open', open_hook,
                                               'open_hook_logger')
        self.run_bzr('switch --create-branch -d checkout feature')
        self.assertEqual(['checkout', 'feature'], opened)

b = ['checkout',
 'checkout',
 'checkout',
 'checkout',
 'checkout',
 'checkout',
 'feature',
 'master',
 'feature',
 'checkout',
 'checkout',
 'checkout',
 'master',
 'feature',
 'checkout']

In theory, we should only open each branch one time. This is capturing Branch.__init__ calls. (via the Branch.open hook).