Checkout existing branch to established workspace (.moved)

[Martin Pool]

I think for things that do an implicit merge (including checkout,
update, and switch) it might be nice to have an option to warn/confirm
if they're going to create conflicts when merged against the tree, or
perhaps even if there are any uncommitted changes.

There is a separate bug pointing out that checking out into a nonempty
directory is a surprising thing to do by default.

m