Function git_ref_invalid_reason 

pub fn git_ref_invalid_reason(branch: &str) -> Option<String>

Check if a string is a valid git branch name. Returns None if valid, or Some(reason) if invalid. Based on git’s check-ref-format rules:

• Cannot contain spaces, tabs, or control characters
• Cannot contain .. (dotdot)
• Cannot contain @{ (at brace)
• Cannot start with . or end with .lock
• Cannot contain /./ or // or end with /
• Cannot be @ or contain @{ (reflog syntax)