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/grep.c

@@ -547,7 +547,7 @@ static int grep_tree(struct grep_opt *opt, const struct pathspec *pathspec,
 	int old_baselen = base->len;
 
 	while (tree_entry(tree, &entry)) {
-		int te_len = tree_entry_len(entry.path, entry.sha1);
+		int te_len = tree_entry_len(&entry);
 
 		if (match != 2) {
 			match = tree_entry_interesting(&entry, base, tn_len, pathspec);