Merge branch 'nd/export-worktree'

Running an aliased command from a subdirectory when the .git thing
in the working tree is a gitfile pointing elsewhere did not work.

* nd/export-worktree:
  setup: set env $GIT_WORK_TREE when work tree is set, like $GIT_DIR

environment.c

@@ -237,6 +237,8 @@ void set_git_work_tree(const char *new_work_tree)
 	}
 	git_work_tree_initialized = 1;
 	work_tree = xstrdup(real_path(new_work_tree));
+	if (setenv(GIT_WORK_TREE_ENVIRONMENT, work_tree, 1))
+		die("could not set GIT_WORK_TREE to '%s'", work_tree);
 }
 
 const char *get_git_work_tree(void)