BUG: show time values in repr of high precision DatetimeIndex
- [ ] closes #19030 
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

On master:
```
In [2]: pd.DatetimeIndex(['2017-01-01 23:59:59.999999999'])
Out[2]:  DatetimeIndex(['2017-01-01'], dtype='datetime64[ns]', freq=None)
```
On my branch:
```
In [3]: pd.DatetimeIndex(['2017-01-01 23:59:59.999999999'])
Out[3]: DatetimeIndex(['2017-01-01 23:59:59.999999999'], dtype='datetime64[ns]', freq=None)
```
It seems that it's only a problem when the time value was 23:59:59.999999999 or very similar, but I may need to add some more tests to check this further.