Add tests for commit, reuse master branch transport.

* bzrlib/workingtree.py:
(WorkingTree.pull): Add a possible_transports parameter, forward
it to branch.pull.

* bzrlib/branch.py:
(Branch.pull, BzrBranch.pull, BzrBranch5.pull): Add a
possible_transports parameter.
(BzrBranch5.pull): Make master branch transport added to
possible_transports

* bzrlib/tests/commands/test_commit.py: 
New file testing against multiple connections during commit.

* bzrlib/builtins.py:
(cmd_pull.run): Allows master branch transports to be reused.
(cmd_commit): 'fixes' is a ListOption, its default value should
be [].


* bzrlib/tests/commands/test_pull.py: 
Fix import statements.
(TestPull.test_pull_with_bound_branch): New test.

* bzrlib/tests/commands/__init__.py:
(test_suite): Add test_commit.