[BUG] TimedeltaIndex intersects with decreasing TimedeltaIndex
-  closes #17391
-  tests added (two tests taken from #22114)
-  passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
-  whatsnew entry

Beside the bug of decreasing TimedeltaIndex, this PR also solve the following
```
In [2]: idx1 = pd.to_timedelta(range(1,20), unit='s')                           

In [3]: idx2 = pd.to_timedelta((1,2,3,4,5,6,7,8,9,10,20), unit='s')             

In [4]: result = idx1.intersection(idx2)                                        

In [5]: result                                                                  
Out[5]: 
TimedeltaIndex(['00:00:01', '00:00:02', '00:00:03', '00:00:04', '00:00:05',
                '00:00:06', '00:00:07', '00:00:08', '00:00:09', '00:00:10',
                '00:00:11', '00:00:12', '00:00:13', '00:00:14', '00:00:15',
                '00:00:16', '00:00:17', '00:00:18', '00:00:19'],
               dtype='timedelta64[ns]', freq=None)

```
The above is also added into the tests.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25661?src=pr&el=h1) Report
> Merging [#25661](https://codecov.io/gh/pandas-dev/pandas/pull/25661?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/c4fa3c93c7cee54249eaffc67d0e5a1e306e967c?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25661/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25661?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25661      +/-   ##
==========================================
+ Coverage   91.29%   91.29%   +<.01%     
==========================================
  Files         173      173              
  Lines       52961    52966       +5     
==========================================
+ Hits        48350    48357       +7     
+ Misses       4611     4609       -2
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.87% <100%> (ø)` | :arrow_up: |
| #single | `41.73% <0%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25661?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/timedeltas.py](https://codecov.io/gh/pandas-dev/pandas/pull/25661/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy90aW1lZGVsdGFzLnB5) | `90.95% <100%> (+0.68%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25661?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25661?src=pr&el=footer). Last update [c4fa3c9...a7e6853](https://codecov.io/gh/pandas-dev/pandas/pull/25661?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25661?src=pr&el=h1) Report
> Merging [#25661](https://codecov.io/gh/pandas-dev/pandas/pull/25661?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/a1fee9199eba7ebf423880243936b9f1501d3d3a?src=pr&el=desc) will **decrease** coverage by `0.52%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25661/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25661?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25661      +/-   ##
==========================================
- Coverage   91.82%   91.29%   -0.53%     
==========================================
  Files         175      173       -2     
  Lines       52551    52966     +415     
==========================================
+ Hits        48256    48357     +101     
- Misses       4295     4609     +314
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.87% <100%> (-0.51%)` | :arrow_down: |
| #single | `41.73% <0%> (+0.87%)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25661?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/timedeltas.py](https://codecov.io/gh/pandas-dev/pandas/pull/25661/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy90aW1lZGVsdGFzLnB5) | `90.95% <100%> (+0.68%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/25661/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-62.5%)` | :arrow_down: |
| [pandas/io/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/25661/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2NvbW1vbi5weQ==) | `72.86% <0%> (-19.01%)` | :arrow_down: |
| [pandas/compat/\_\_init\_\_.py](https://codecov.io/gh/pandas-dev/pandas/pull/25661/diff?src=pr&el=tree#diff-cGFuZGFzL2NvbXBhdC9fX2luaXRfXy5weQ==) | `58.03% <0%> (-17.8%)` | :arrow_down: |
| [pandas/io/excel/\_util.py](https://codecov.io/gh/pandas-dev/pandas/pull/25661/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2V4Y2VsL191dGlsLnB5) | `78.82% <0%> (-8.68%)` | :arrow_down: |
| [pandas/compat/chainmap.py](https://codecov.io/gh/pandas-dev/pandas/pull/25661/diff?src=pr&el=tree#diff-cGFuZGFzL2NvbXBhdC9jaGFpbm1hcC5weQ==) | `61.9% <0%> (-4.77%)` | :arrow_down: |
| [pandas/core/groupby/categorical.py](https://codecov.io/gh/pandas-dev/pandas/pull/25661/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9jYXRlZ29yaWNhbC5weQ==) | `95.45% <0%> (-4.55%)` | :arrow_down: |
| [pandas/core/computation/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/25661/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tcHV0YXRpb24vY29tbW9uLnB5) | `85.71% <0%> (-3.76%)` | :arrow_down: |
| [pandas/core/dtypes/cast.py](https://codecov.io/gh/pandas-dev/pandas/pull/25661/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2Nhc3QucHk=) | `88.16% <0%> (-3.2%)` | :arrow_down: |
| [pandas/io/s3.py](https://codecov.io/gh/pandas-dev/pandas/pull/25661/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3MzLnB5) | `86.36% <0%> (-3.12%)` | :arrow_down: |
| ... and [118 more](https://codecov.io/gh/pandas-dev/pandas/pull/25661/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25661?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25661?src=pr&el=footer). Last update [a1fee91...90d32f2](https://codecov.io/gh/pandas-dev/pandas/pull/25661?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 can you merge master