unpack-trees: preserve cache_bottom

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>