split-index: do not invalidate cache-tree at read time

We are sure that after merge_base_index() is done. cache-tree can
still be used with the final index. So don't destroy cache tree.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

split-index.c

@@ -133,6 +133,7 @@ void merge_base_index(struct index_state *istate)
 	for (i = si->nr_replacements; i < si->saved_cache_nr; i++) {
 		add_index_entry(istate, si->saved_cache[i],
 				ADD_CACHE_OK_TO_ADD |
+				ADD_CACHE_KEEP_CACHE_TREE |
 				/*
 				 * we may have to replay what
 				 * merge-recursive.c:update_stages()