BUG: to_datetime inconsistent output container type
#### Code Sample, a copy-pastable example if possible

```python
>>> import pandas as pd
>>> type(pd.to_datetime(pd.Categorical(['2019-09-24 17:00:00']*50)))
<class 'pandas.core.indexes.datetimes.DatetimeIndex'>
>>> type(pd.to_datetime(pd.Categorical(['2019-09-24 17:00:00']*51)))
<class 'pandas.core.indexes.category.CategoricalIndex'>
```
#### Problem description

The output container type is inconsistent for `pd.to_datetime` and flips once the series is above 50 rows. This is an issue since the `CategoricalIndex` class does not support DateTime-like operations (tz_localize, stftime, etc.). 
Since the behaviour switches between differently sized Series objects this is particularly troublesome since code behaves quite different depending on the amount of data we feed it (e.g. unit test vs. production).

#### Expected Output

Consistent container type.

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.6.6.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.9.184-linuxkit
machine          : x86_64
processor        :
byteorder        : little
LC_ALL           : C.UTF-8
LANG             : C.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 0.25.1
numpy            : 1.17.0
pytz             : 2019.2
dateutil         : 2.8.0
pip              : 19.2.2
setuptools       : 39.1.0
Cython           : None
pytest           : 5.1.1
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : 1.0.5
lxml.etree       : 4.4.1
html5lib         : None
pymysql          : None
psycopg2         : 2.8.3 (dt dec pq3 ext lo64)
jinja2           : 2.10.1
IPython          : 7.7.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.4.1
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : 2.5.4
pandas_gbq       : None
pyarrow          : 0.13.0
pytables         : None
s3fs             : None
scipy            : None
sqlalchemy       : 1.3.7
tables           : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : 1.0.5
</details>
