diff-lib.c: don't strdup twice

The static function read_directory in diff-lib.c is only ever called
with struct path_list lists with .strdup_paths turned on, i.e.
path_list_insert will strdup the paths for us (again).  Let's take
advantage of that and stop doing it twice.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff-lib.c

@@ -24,7 +24,7 @@ static int read_directory(const char *path, struct path_list *list)
 
 	while ((e = readdir(dir)))
 		if (strcmp(".", e->d_name) && strcmp("..", e->d_name))
-			path_list_insert(xstrdup(e->d_name), list);
+			path_list_insert(e->d_name, list);
 
 	closedir(dir);
 	return 0;