Function update_ref 

pub fn update_ref(
    mkit_dir: &Path,
    branch: &str,
    condition: RefWriteCondition,
    h: &Hash,
) -> RefResult<()>

CAS-aware ref write per SPEC-REFS §5.

Errors

• RefError::InvalidRefName if branch is not a valid name.
• RefError::Conflict if condition is not satisfied.
• RefError::Io for filesystem failures.