An earlier change in 2.5.x-era broke users' hooks and aliases by
exporting GIT_WORK_TREE to point at the root of the working tree,
interfering when they tried to use a different working tree without
setting GIT_WORK_TREE environment themselves.

* nd/stop-setenv-work-tree:
  Revert "setup: set env $GIT_WORK_TREE when work tree is set, like $GIT_DIR"