tree-walk.c: do not leak internal structure in tree_entry_len()

tree_entry_len() does not simply take two random arguments and return
a tree length. The two pointers must point to a tree item structure,
or struct name_entry. Passing random pointers will return incorrect
value.

Force callers to pass struct name_entry instead of two pointers (with
hope that they don't manually construct struct name_entry themselves)

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/pack-objects.c

@@ -979,7 +979,7 @@ static void add_pbase_object(struct tree_desc *tree,
 	while (tree_entry(tree,&entry)) {
 		if (S_ISGITLINK(entry.mode))
 			continue;
-		cmp = tree_entry_len(entry.path, entry.sha1) != cmplen ? 1 :
+		cmp = tree_entry_len(&entry) != cmplen ? 1 :
 		      memcmp(name, entry.path, cmplen);
 		if (cmp > 0)
 			continue;