Fix bug in contains when looking up a string in a non-monotonic datet…
- [ ] closes #13572
- [x] tests added / passed
  pandas.tests.indexes.test_datetimelike.TestDatetimeIndex:test_contains
- [ ] passes `git diff upstream/master | flake8 --diff`
- [ ] whatsnew entry

…ime index and the object in question is first in the index.

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13574?src=pr) is **84.31%**

> Merging [#13574](https://codecov.io/gh/pydata/pandas/pull/13574?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will decrease coverage by **0.03%**

``` diff
@@             master     #13574   diff @@
==========================================
  Files           138        138          
  Lines         51126      51157    +31   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43122      43132    +10   
- Misses         8004       8025    +21   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [d38ee27...3c202b1](https://codecov.io/gh/pydata/pandas/compare/d38ee272f3060cb884f21f9f7d212efc5f7656a8...3c202b1cbcc73c4006c967c8abe1b8d9089c5be4?src=pr)
 needs a rebase. some of these tests may now be passing. #13609 fixed lots of things.
 Hmm, I think I made a mistake in rebasing. Looks like I pulled in every single change in to this pull request. I'll close this one and make a new one.
