Merge branch 'dg/submodule-in-dismembered-working-tree'

In a superproject that has repository outside of its working tree,
"git submodule add" failed to clone a new submodule, as GIT_DIR and
GIT_WORK_TREE environment variables necessary to work in such a
superproject interfered with access to the submodule repository.

* dg/submodule-in-dismembered-working-tree:
  git-submodule: work with GIT_DIR/GIT_WORK_TREE