Extend tests for checkout with typechanges

Extend tests for checkout with typechanges

Test a number of other cases, including intentionally forced
conflicts and deeper inspection that trees get created properly.

There is a still a bug in checkout because the first test here
(i.e. test_checkout_typechange__checkout_typechanges_safe) should
be able to pass with GIT_CHECKOUT_SAFE as a strategy, but it will
not because of some lingering submodule checkout issues.