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>