tree-walk: replace hard-coded constants with the_hash_algo

Remove the hard-coded 20-based values and replace them with uses of
the_hash_algo.

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

tree-walk.c

@@ -26,8 +26,9 @@ static int decode_tree_entry(struct tree_desc *desc, const char *buf, unsigned l
 {
 	const char *path;
 	unsigned int mode, len;
+	const unsigned hashsz = the_hash_algo->rawsz;
 
-	if (size < 23 || buf[size - 21]) {
+	if (size < hashsz + 3 || buf[size - (hashsz + 1)]) {
 		strbuf_addstr(err, _("too-short tree object"));
 		return -1;
 	}