* bignum.c (bary_subb): Support xn < yn.
  (bigsub_core): Removed.
  (bigsub): Don't compare before subtraction.  Just subtract and
  get the two's complement if the subtraction causes a borrow.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41962 b2dd03c8-39d4-4d8f-98ff-823fe69b080e