BUG: TimedeltaIndex.intersection fails for decreasing TimedeltaIndex
#### Code Sample, a copy-pastable example if possible
Setup
```python
In [1]: import pandas as pd
   ...: idx1 = pd.to_timedelta(range(3), unit='s')
   ...: idx2 = pd.to_timedelta(range(2, -1, -1), unit='s')
   ...:

In [2]: idx1
Out[2]: TimedeltaIndex(['00:00:00', '00:00:01', '00:00:02'], dtype='timedelta64[ns]', freq=None)

In [3]: idx2
Out[3]: TimedeltaIndex(['00:00:02', '00:00:01', '00:00:00'], dtype='timedelta64[ns]', freq=None)
```

Intersection returns empty between `idx1` and `idx2`:
```python
In [4]: idx1.intersection(idx2)
Out[4]: TimedeltaIndex([], dtype='timedelta64[ns]', freq=None)

In [5]: idx2.intersection(idx1)
Out[5]: TimedeltaIndex([], dtype='timedelta64[ns]', freq=None)
```

Even an intersection of `idx2` with itself returns empty:
```python
In [6]: idx2.intersection(idx2)
Out[6]: TimedeltaIndex([], dtype='timedelta64[ns]', freq=None)
```

#### Problem description
The intersections are returning empty even though all elements are shared.

#### Expected Output
I'd expect all of the intersections to return a `TimedeltaIndex` containing all three elements.

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.2.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 78 Stepping 3, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.20.3
pytest: 3.1.2
pip: 9.0.1
setuptools: 27.2.0
Cython: 0.26
numpy: 1.13.1
scipy: 0.19.1
xarray: None
IPython: 6.1.0
sphinx: 1.5.6
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2017.2
blosc: None
bottleneck: 1.2.1
tables: 3.2.2
numexpr: 2.6.2
feather: None
matplotlib: 2.0.2
openpyxl: 2.4.7
xlrd: 1.0.0
xlwt: 1.2.0
xlsxwriter: 0.9.6
lxml: 3.7.3
bs4: 4.6.0
html5lib: 0.999
sqlalchemy: 1.1.9
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
pandas_gbq: None
pandas_datareader: None

</details>
