code: function revExists(revs, rev) {
    var toVisit = revs.slice();
    var splitRev = rev.split('-');
    var targetPos = parseInt(splitRev[0], 10);
    var targetId = splitRev[1];
  
    var node;
    while ((node = toVisit.pop())) {
      if (node.pos === targetPos && node.ids[0] === targetId) {
        return true;
      }
      var branches = node.ids[2];
      for (var i = 0, len = branches.length; i < len; i++) {
        toVisit.push({pos: node.pos + 1, ids: branches[i]});
      }
    }
    return false;
  }
docs: return true if a rev exists in the rev tree, false otherwise