Function diff_trees 

pub fn diff_trees<S: ObjectStore + ?Sized>(
    store: &S,
    from: &ContentHash,
    to: &ContentHash,
) -> Result<FileChangeSet, Error>

Collect all file changes between two trees.

This is the materializing variant: it walks the trees via diff_trees_visit and collects every FileChange into a FileChangeSet. Streaming or early-exit consumers should prefer diff_trees_visit, which avoids allocating the full change list.