The core.worktree setting in a submodule repository should not be
pointing at a directory when the submodule loses its working tree
(e.g. getting deinit'ed), but the code did not properly maintain
this invariant.
* sb/submodule-unset-core-worktree-when-worktree-is-lost:
submodule deinit: unset core.worktree
submodule--helper: fix BUG message in ensure_core_worktree
submodule: unset core.worktree if no working tree is present
submodule update: add regression test with old style setups