Code review comment for lp:~spiv/bzr/default-stacking-again

Fix for bug 388675. When using the BzrDirFormat.initialize_ex_1.16 verb the wrong branch format was created by the client if stacking was triggered by a default stacking policy. The cause was that in that case require_stacking() wasn't being triggered in the client, just server-side, but it's the client that chooses the branch format.

So this patch simply makes sure require_stacking() is called if the server requires stacking. This may be a candidate for cherrypicking into 1.16...