Merge branch 'sb/submodule-unset-core-worktree-when-worktree-is-lost'

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