Merge branch 'nd/stop-setenv-work-tree' into maint

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"