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

diff --cc builtin/submodule--helper.c
Simple merge