read-cache: mark updated entries for split index

The large part of this patch just follows CE_ENTRY_CHANGED
marks. replace_index_entry() is updated to update
split_index->base->cache[] as well so base->cache[] does not reference
to a freed entry.

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

cache.h

@@ -169,6 +169,8 @@ struct cache_entry {
 /* used to temporarily mark paths matched by pathspecs */
 #define CE_MATCHED           (1 << 26)
 
+#define CE_UPDATE_IN_BASE    (1 << 27)
+
 /*
  * Extended on-disk flags
  */