repr for PeriodIndex does not handle <=2 elements well (GH5372)
closes https://github.com/pydata/pandas/issues/5372

pls try to name the issues and commits to something related to the issue (and just reference the issue, e.g. GH5372)
 @jreback sorry, changed name to more appropriate
 gr8
 can you add a release notes entry refing this issue? (doc/source/release.rst)
 pls also rebase and squash: https://github.com/pydata/pandas/wiki/Using-Git
 @jreback  hope I did it right
 the release notes look fine, but you need to squash...

```
git rebase -i <commit hash before your first one>
```

then `s` or `f` on the commits

then 

```
git push myfork thisbranch -f
```
 @jreback another try, sorry, used github only occasionally before
 @RomanPekar thanks!
