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.
