index: future proof for "extended" index entries

We do not have any more bits in the on-disk index flags word, but we would
need to have more in the future.  Use the last remaining bits as a signal
to tell us that the index entry we are looking at is an extended one.

Since we do not understand the extended format yet, we will just error out
when we see it.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

cache.h

@@ -126,6 +126,7 @@ struct cache_entry {
 
 #define CE_NAMEMASK  (0x0fff)
 #define CE_STAGEMASK (0x3000)
+#define CE_EXTENDED  (0x4000)
 #define CE_VALID     (0x8000)
 #define CE_STAGESHIFT 12
 

read-cache.c

@@ -1118,6 +1118,10 @@ static void convert_from_disk(struct ondisk_cache_entry *ondisk, struct cache_en
 	ce->ce_size  = ntohl(ondisk->size);
 	/* On-disk flags are just 16 bits */
 	ce->ce_flags = ntohs(ondisk->flags);
+
+	/* For future extension: we do not understand this entry yet */
+	if (ce->ce_flags & CE_EXTENDED)
+		die("Unknown index entry format");
 	hashcpy(ce->sha1, ondisk->sha1);
 
 	len = ce->ce_flags & CE_NAMEMASK;