Refactor mutiple connections detection and fix false positives. Only
merge and pull are still bougs.

* bzrlib/tests/commands/test_push.py:
(TestPush.test_push): Reset connections after branch and tree
creation.

* bzrlib/tests/commands/test_missing.py:
(TestMissing.test_missing): Reset connections after branch and tree
creation.

* bzrlib/tests/commands/test_merge.py:
(TestMerge.test_merge): Reset connections after branch and tree
creation.

* bzrlib/tests/commands/test_checkout.py:
(TestCheckout.test_checkout): Reset connections after branch and tree
creation.

* bzrlib/tests/commands/test_cat.py:
(TestCat.test_cat): Reset connections after branch and tree
creation.

* bzrlib/tests/commands/test_branch.py:
(TestBranch.setUp): Reset connections after branch and tree
creation.

* bzrlib/tests/TransportUtil.py:
(TransportHooks.__init__): Use _set_connection instead of
_get_FTP.
(ConnectionHookedTransport): Replace _get_FTP by _set_connection.
(TestCaseWithConnectionHookedTransport.setUp): Replace _get_FTP by
_set_connection.
(TestCaseWithConnectionHookedTransport.reset_connections): New
method.
(TestCaseWithConnectionHookedTransport.set_connection_hook):
Replace get_connection_hook.