unpack-trees: take care to propagate the split-index flag

When copying the `split_index` structure from one index structure to
another, we need to propagate the `SPLIT_INDEX_ORDERED` flag, too, if it
is set, otherwise Git might forget to write the shared index when that
is actually needed.

It just so _happens_ that in many instances when `unpack_trees()` is
called, the result causes the shared index to be written anyway, but
there are edge cases when that is not so.

Signed-off-by: Johannes Schindelin <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: dscho

unpack-trees.c

@@ -1916,6 +1916,8 @@ int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options
 		 * create a new one.
 		 */
 		o->result.split_index = o->src_index->split_index;
+		if (o->src_index->cache_changed & SPLIT_INDEX_ORDERED)
+			o->result.cache_changed |= SPLIT_INDEX_ORDERED;
 		o->result.split_index->refcount++;
 	} else {
 		o->result.split_index = init_split_index(&o->result);