difftool: remove obsolete (and misleading) comment

We will always spawn something from `git difftool`, so we will always
have to set `GIT_DIR` and `GIT_WORK_TREE`.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/difftool.c

@@ -727,7 +727,6 @@ int cmd_difftool(int argc, const char **argv, const char *prefix)
 	if (tool_help)
 		return print_tool_help();
 
-	/* NEEDSWORK: once we no longer spawn anything, remove this */
 	setenv(GIT_DIR_ENVIRONMENT, absolute_path(get_git_dir()), 1);
 	setenv(GIT_WORK_TREE_ENVIRONMENT, absolute_path(get_git_work_tree()), 1);