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>

tree.c

@@ -99,7 +99,7 @@ static int read_tree_1(struct tree *tree, struct strbuf *base,
 		else
 			continue;
 
-		len = tree_entry_len(entry.path, entry.sha1);
+		len = tree_entry_len(&entry);
 		strbuf_add(base, entry.path, len);
 		strbuf_addch(base, '/');
 		retval = read_tree_1(lookup_tree(sha1),