Function delete_reference_if_present 

pub fn delete_reference_if_present(
    repo: &Repository,
    name: &str,
) -> GitProjectionResult<()>

Delete a reference if present; missing-ref is a no-op. Used by the write-through rollback path to drop a branch that was created by a failed write-through but isn’t reachable from any prior state. We scope the deletion with RefPrecondition::MustExist so an unrelated concurrent writer that just updated this ref isn’t silently clobbered — if the ref vanished underneath us between our read and the delete, that’s the rollback we wanted anyway.