While there may not be a known, direct, attack against git, WebKit's SVN repository was demonstrably attacked by the SHA-1 vulnerability, oddly by their own developers today (or yesterday?). That it took that little time from a PoC to an actual production issue leads me to believe it wouldn't take long for a dedicated individual or team to extend the vulnerability to git and it's "mitigation" involving the header.

I prefer not to take the bury my head in the sand approach to this, at least when it comes to public repositories.