Function diff_tree_path_change_count 

pub fn diff_tree_path_change_count(
    odb: &Odb,
    old_tree_oid: Option<&ObjectId>,
    new_tree_oid: Option<&ObjectId>,
    prefix: &str,
) -> Result<usize>

Count path-level differences between two trees without building a DiffEntry list.

Same semantics as diff_trees, but avoids allocating diff records (issue #1064 commit hot path).

Errors

Returns errors from object database reads.