BUG: date slicing with reverse sorted index
- [x] closes #14316
- [ ] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

cc @shoyer - good diagnosis!

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/14317?src=pr) is 85.26% (diff: 100%)

> Merging [#14317](https://codecov.io/gh/pydata/pandas/pull/14317?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #14317   diff @@
==========================================
  Files           140        140          
  Lines         50619      50621     +2   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43161      43163     +2   
  Misses         7458       7458          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [5033a4a...04feea1](https://codecov.io/gh/pydata/pandas/compare/5033a4a799c77fdc7e868a9f332384eabcc332b8...04feea1e75d34119008f21d704880137af5d5292?src=pr)
 lgtm. minor comment. merge when ready.
 thanks!
