pack-objects: don't include missing preferred base objects

This improves commit 6d6f9cddbe a bit by simply not including missing
bases in the list of objects to process at all.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-pack-objects.c

@@ -1725,6 +1725,14 @@ static void prepare_pack(int window, int depth)
 			if (entry->type < 0)
 				die("unable to get type of object %s",
 				    sha1_to_hex(entry->idx.sha1));
+		} else {
+			if (entry->type < 0) {
+				/*
+				 * This object is not found, but we
+				 * don't have to include it anyway.
+				 */
+				continue;
+			}
 		}
 
 		delta_list[n++] = entry;