PERF: unecessary sort/concat in core/internals/combine when there's 1 block
So combine with blocks==1 does unecessary work, right [here](https://github.com/pydata/pandas/blob/master/pandas/core/internals.py#L3002)

can be seen vs 0.17.1 from [SO](http://stackoverflow.com/questions/36274447/pd-rolling-mean-becoming-deprecated-alternatives-for-ndarrays/36274775#36274775)

```
import pandas as pd
import numpy as np

s = pd.Series(np.random.normal(size=100000))
s.rolling(window=2).mean()
```
