bzr reconfigure --stacked-on should check new fallback has all necessary data

"bzr reconfigure --stacked-on URL", when switching from some previous fallback URL, should check that the new fallback URL has all the necessary data, and if it does not, fetch from the old fallback URL, similarly to what it does when unstacking.

In some cases, the user may be doing this because the old fallback branch has moved, therefore it can no longer be opened. We should either detect that and not try to copy, or perhaps have an option to avoid checking, which could possibly be useful anyhow because the check may take some time.

This is a follow-on to bug 391411 suggested in <https://code.edge.launchpad.net/~mbp/bzr/391411-reconfigure-stacked/+merge/8527>.