Function atomic_replace_commit_graph 

pub fn atomic_replace_commit_graph(
    objects_dir: &Path,
    bytes: &[u8],
) -> Result<PathBuf>

Replace objects/info/commit-graph atomically (lock + tempfile + rename).

Does not remove a split commit-graphs/ chain; callers use remove_commit_graph_chain_dir after a successful monolithic write.

Errors

Propagates I/O failures or when the lock file is already held.