tree-walk: avoid hard-coded 20 constant

Use the_hash_algo to look up the length of our current hash instead of
hard-coding the value 20.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

tree-walk.c

@@ -105,7 +105,7 @@ static void entry_extract(struct tree_desc *t, struct name_entry *a)
 static int update_tree_entry_internal(struct tree_desc *desc, struct strbuf *err)
 {
 	const void *buf = desc->buffer;
-	const unsigned char *end = desc->entry.oid->hash + 20;
+	const unsigned char *end = desc->entry.oid->hash + the_hash_algo->rawsz;
 	unsigned long size = desc->size;
 	unsigned long len = end - (const unsigned char *)buf;