Function diff_nondestructive 

pub fn diff_nondestructive(
    obj_store: &mut GitObjectStore,
    commits_table: &CommitsTable,
    base_commit_id: &str,
    head_commit_id: &str,
) -> Result<DiffResult, CommitError>

Compute diff without mutating the store — saves and restores current state.

Use this when you have uncommitted changes you want to preserve.