sha1_object_info_extended(): hint about objects in delta-base cache

An object found in the delta-base cache is not guaranteed to
stay there, but we know it came from a pack and it is likely
to give us a quick access if we read_sha1_file() it right now,
which is a piece of useful information.

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

cache.h

@@ -1030,7 +1030,8 @@ struct object_info {
 	enum {
 		OI_CACHED,
 		OI_LOOSE,
-		OI_PACKED
+		OI_PACKED,
+		OI_DBCACHED
 	} whence;
 	union {
 		/*

sha1_file.c

@@ -1697,6 +1697,13 @@ static unsigned long pack_entry_hash(struct packed_git *p, off_t base_offset)
 	return hash % MAX_DELTA_CACHE;
 }
 
+static int in_delta_base_cache(struct packed_git *p, off_t base_offset)
+{
+	unsigned long hash = pack_entry_hash(p, base_offset);
+	struct delta_base_cache_entry *ent = delta_base_cache + hash;
+	return (ent->data && ent->p == p && ent->base_offset == base_offset);
+}
+
 static void *cache_or_unpack_entry(struct packed_git *p, off_t base_offset,
 	unsigned long *base_size, enum object_type *type, int keep_cache)
 {
@@ -2128,6 +2135,8 @@ int sha1_object_info_extended(const unsigned char *sha1, struct object_info *oi)
 	if (status < 0) {
 		mark_bad_packed_object(e.p, sha1);
 		status = sha1_object_info_extended(sha1, oi);
+	} else if (in_delta_base_cache(e.p, e.offset)) {
+		oi->whence = OI_DBCACHED;
 	} else {
 		oi->whence = OI_PACKED;
 		oi->u.packed.offset = e.offset;