unpack-trees: preserve cache_bottom

The cache_bottom member of 'struct unpack_trees_options' is used to
track the range of index entries corresponding to a node of the cache
tree. While recursing with traverse_by_cache_tree(), this value is
preserved on the call stack using a local and then restored as that
method returns.

The mark_ce_used() method normally modifies the cache_bottom member when
it refers to the marked cache entry. However, sparse directory entries
are stored as nodes in the cache-tree data structure as of 2de37c53
(cache-tree: integrate with sparse directory entries, 2021-03-30). Thus,
the cache_bottom will be modified as the cache-tree walk advances. Do
not update it as well within mark_ce_used().

Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

unpack-trees.c

@@ -600,6 +600,13 @@ static void mark_ce_used(struct cache_entry *ce, struct unpack_trees_options *o)
 {
 	ce->ce_flags |= CE_UNPACKED;
 
+	/*
+	 * If this is a sparse directory, don't advance cache_bottom.
+	 * That will be advanced later using the cache-tree data.
+	 */
+	if (S_ISSPARSEDIR(ce->ce_mode))
+		return;
+
 	if (o->cache_bottom < o->src_index->cache_nr &&
 	    o->src_index->cache[o->cache_bottom] == ce) {
 		int bottom = o->cache_bottom;