concat with ExtensionArray and object upcasting fails
```python
In [1]: import pandas as pd

In [2]: from pandas.tests.extension.decimal.array import *

In [3]: result = pd.concat([pd.Series(DecimalArray([decimal.Decimal(1)])), pd.Series([1])])

In [4]: result.shape
Out[4]: (2, 1)
```

and the repr fails, since things are messed up.