Code review comment for lp:~garyvdm/bzr/get_trees_and_branches_to_diff

I agree with John about bzrdir.BzrDir.open_containing_tree_or_branch(new_url)
 not accepting a possible_transport parameter.

On the other hand, I'm not convinced that we can blindly share connections
for two potentially concurrent operations and, since we don't have the test
infrastructure for such assertions, that's out of scope for that patch.

I'll merge.