discard_cache: reset lazy name_hash bit

We forgot to reset name_hash_initialized bit when discarding the in-core index.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

read-cache.c

@@ -1243,6 +1243,7 @@ int discard_index(struct index_state *istate)
 	istate->cache_nr = 0;
 	istate->cache_changed = 0;
 	istate->timestamp = 0;
+	istate->name_hash_initialized = 0;
 	free_hash(&istate->name_hash);
 	cache_tree_free(&(istate->cache_tree));
 	free(istate->alloc);