function diff_bisectSplit_(text1, text2, x, y) {
  var text1a = text1.substring(0, x);
  var text2a = text2.substring(0, y);
  var text1b = text1.substring(x);
  var text2b = text2.substring(y);

  // Compute both diffs serially.
  var diffs = diff_main(text1a, text2a);
  var diffsb = diff_main(text1b, text2b);

  return diffs.concat(diffsb);
}
Given the location of the 'middle snake', split the diff in two parts
and recurse.
@param {string} text1 Old string to be diffed.
@param {string} text2 New string to be diffed.
@param {number} x Index of split point in text1.
@param {number} y Index of split point in text2.
@return {Array} Array of diff tuples.