BUG: DataFrame.truncate() returns incorrect data if the index is sorted in descending order
- [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 (1.0.3)

- [ ] (optional) I have confirmed this bug exists on the master branch of pandas.

---

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

```python
df = pd.DataFrame(index=range(0, 5), data={'A': 1})
df.sort_index(ascending=False, inplace=True)
df.truncate(before=3)
```

#### Problem description

When the index is sorted in descending order, the truncate() function behaves incorrectly. In the example above, it should truncate all rows whose index is < 3. Instead it is truncating all rows whose index is > 3.
Similarly when using after.

#### Output

```python
>>> df
   A
4  1
3  1
2  1
1  1
0  1
>>> df.truncate(before=3)
   A
3  1
2  1
1  1
0  1
```

#### Expected Output

```python
>>> df.truncate(before=3)
   A
4  1
3  1
```

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

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.7.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 19.4.0
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.0.3
numpy            : 1.18.2
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 20.0.2
setuptools       : 41.2.0
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       : 3.2.1
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>
