BUG: DatetimeIndex.get_indexer wrong result on mixed-type argument
- [x] I have checked that this issue has not already been reported.
- [x] I have confirmed this bug exists on the latest version of pandas.
- [ ] (optional) I have confirmed this bug exists on the master branch of pandas.

#### Code Sample, a copy-pastable example

```python
import datetime
import pandas as pd

dates = pd.bdate_range('2020-04-20', '2020-04-25')
print(dates.get_indexer([datetime.date(2020, 4, 20), datetime.date(2020, 4, 21)]))
# [0, 1]
print(dates.get_indexer([pd.Timestamp('2020-04-20'), datetime.date(2020, 4, 21)]))
# [0, -1]
```

#### Problem description

As a general rule I would expect that `index.get_indexer(as + bs) == np.concat([index.get_indexer(as), index.get_indexer(bs)])` for any lists `as` and `bs`, i.e. `index.get_indexer` operates elementwise. However in this case the type of a previous argument is surprisingly affecting the index that is assigned to a later one.

#### Expected Output

`[0, 1]` in both cases

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

<details>
INSTALLED VERSIONS
------------------
commit           : None
python           : 3.8.2.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           : 1.0.3
numpy            : 1.18.1
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 20.0.2
setuptools       : 46.1.3.post20200330
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>
