Fix second unwanted connection by providing the right branch to create_checkout.

* bzrlib/branch.py:
(Branch.create_checkout): Keep the lightweight branch to create
the checkout from.

* bzrlib/bzrdir.py:
(BzrDir.create_workingtree, BzrDir.open_workingtree,
BzrDirMeta1.create_workingtree): Add a from_branch parameter for
lightweight checkouts purposes.

* bzrlib/workingtree.py:
(WorkingTreeFormat2.initialize, WorkingTreeFormat3.initialize):
Use from_branch if provided or default to bzrdir branch.

* bzrlib/workingtree_4.py:
(WorkingTreeFormat4.initialize): Use from_branch if provided or
default to bzrdir branch.

* bzrlib/tests/test_workingtree.py:
(SampleTreeFormat.initialize): Update the signature.

* bzrlib/remote.py:
(RemoteBzrDir.create_workingtree): Update the signature.