pack-objects: tweak "do not even attempt delta" heuristics

The heuristics to give up deltification when both the source and the
target are both in the same pack affects negatively when we are
repacking the subset of objects in the existing pack.  This caused
any incremental updates to use suboptimal packs.  Tweak the heuristics
to avoid this problem.

Signed-off-by: Junio C Hamano <junkio@cox.net>

builtin-pack-objects.c

@@ -1176,7 +1176,9 @@ static int try_delta(struct unpacked *trg, struct unpacked *src,
 	 * on an earlier try, but only when reusing delta data.
 	 */
 	if (!no_reuse_delta && trg_entry->in_pack &&
-	    trg_entry->in_pack == src_entry->in_pack)
+	    trg_entry->in_pack == src_entry->in_pack &&
+	    trg_entry->in_pack_type != OBJ_REF_DELTA &&
+	    trg_entry->in_pack_type != OBJ_OFS_DELTA)
 		return 0;
 
 	/*