Possible wrong global histogram after merging the partition statistics

[winoros]

After the slices.SortFunc, The order like [2, 2], [1, 3], [2, 3] will be a valid sorted result. And the following merging process just checks the bucket size. So it's possible that we treat the [2, 2] as the first global bucket and merge [1, 3] and [2, 3] as the second.

The merge result is wrong.