fix F.repeat does not support negative axis
fix F.repeat does not support negative axis

hvy/chainer,ktnyt/chainer,pfnet/chainer,chainer/chainer,tkerola/chainer,hvy/chainer,hvy/chainer,anaruse/chainer,rezoo/chainer,aonotas/chainer,okuta/chainer,ronekko/chainer,niboshi/chainer,keisuke-umezawa/chainer,jnishi/chainer,keisuke-umezawa/chainer,ktnyt/chainer,ktnyt/chainer,okuta/chainer,ktnyt/chainer,wkentaro/chainer,wkentaro/chainer,jnishi/chainer,wkentaro/chainer,okuta/chainer,niboshi/chainer,jnishi/chainer,keisuke-umezawa/chainer,wkentaro/chainer,niboshi/chainer,okuta/chainer,chainer/chainer,hvy/chainer,keisuke-umezawa/chainer,chainer/chainer,jnishi/chainer,chainer/chainer,niboshi/chainer