`DataFrame.diff` on `datetime64` returns result inconsistent with `Series.diff` 
#### Code Sample, a copy-pastable example if possible

```python
>>> pd.Series(['NaT', '2019-01-01', '2019-01-02'], dtype='datetime64[ns]').diff()
0      NaT
1      NaT
2   1 days
dtype: timedelta64[ns]

>>> pd.Series(['NaT', '2019-01-01', '2019-01-02'], dtype='datetime64[ns]').to_frame().diff()
                             0
0                          NaT
1 -88855 days +00:12:43.145224
2              1 days 00:00:00
```

#### Problem description

Doing `.diff()` on a `DataFrame` of `datetime64` should yield exactly time deltas in exactly the same way as on the `Series` i.e. index `(1,0)` in the `DataFrame` above should be `NaT`, not an overflown timedelta64.

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

<details>
INSTALLED VERSIONS
------------------
commit           : None
python           : 3.8.1.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
machine          : AMD64
processor        : Intel64 Family 6 Model 63 Stepping 2, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : English_United Kingdom.936

pandas           : 1.0.1
numpy            : 1.18.1
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 20.0.2
setuptools       : 45.2.0.post20200210
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : None
IPython          : None
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : None
pyxlsb           : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
numba            : None
</details>
