pack-objects: more threaded load balancing fix with often changed paths

The code that splits the object list amongst work threads tries to do so
on "path" boundaries not to prevent good delta matches.  However, in
some cases, a few paths may largely dominate the hash distribution and
it is not possible to have good load balancing without ignoring those
boundaries.

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

builtin-pack-objects.c

@@ -1709,6 +1709,16 @@ static void ll_find_deltas(struct object_entry **list, unsigned list_size,
 				list++;
 				sub_size--;
 			}
+			if (!sub_size) {
+				/*
+				 * It is possible for some "paths" to have
+				 * so many objects that no hash boundary
+				 * might be found.  Let's just steal the
+				 * exact half in that case.
+				 */
+				sub_size = victim->remaining / 2;
+				list -= sub_size;
+			}
 			target->list = list;
 			victim->list_size -= sub_size;
 			victim->remaining -= sub_size;