lazy index hashing

This delays the hashing of index names until it becomes necessary for
the first time.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -191,6 +191,7 @@ struct index_state {
 	struct cache_tree *cache_tree;
 	time_t timestamp;
 	void *alloc;
+	unsigned name_hash_initialized : 1;
 	struct hash_table name_hash;
 };