LONDON (Reuters) - 1/BACK TO ZEROFew people will be brave enough to call the end of the 30-year bond bullrun, but the idea that sub-zero borrowing costs may not be around forever is one people are increasingly willing to entertain. Upbeat economic data, signs of a trade war truce and above all, central banks' clear reluctance to cut interest rates further have shrunk the global negative-yield bond pool to $12.5 trillion from $17 trillion two months ago.In Europe, Germany's long-dated government..