BUG: Fix group index calculation to prevent hitting maximum recursion depth (#21524)
This just replaces tail recursion call with a simple loop. It should have no effect whatsoever on a performance but prevent hitting recursion limits on some input data ( See example in my issue here: https://github.com/pandas-dev/pandas/issues/21524 )

- [x] closes #21524 
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

3.5 failure in Travis looks like caused by a network glitch :( need to rerun it
```
3.5 is not installed; attempting download
Downloading archive: https://s3.amazonaws.com/travis-python-archives/binaries/ubuntu/14.04/x86_64/python-3.5.tar.bz2
$ curl -sSf -o python-3.5.tar.bz2 ${archive_url}
curl: (56) SSL read: error:00000000:lib(0):func(0):reason(0), errno 104
Unable to download 3.5 archive. The archive may not exist. Please consider a different version.
``` can you add your example as a test Hello @Templarrr! Thanks for updating the PR.





Cheers ! There are no PEP8 issues in this Pull Request. :beers: 

##### Comment last updated on June 20, 2018 at 10:25 Hours UTC # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21541?src=pr&el=h1) Report
> Merging [#21541](https://codecov.io/gh/pandas-dev/pandas/pull/21541?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/2625759abeb78655558067d55a23c293628c3165?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21541/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/21541?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #21541      +/-   ##
==========================================
+ Coverage   91.91%   91.92%   +<.01%     
==========================================
  Files         153      153              
  Lines       49546    49564      +18     
==========================================
+ Hits        45542    45560      +18     
  Misses       4004     4004
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.32% <100%> (ø)` | :arrow_up: |
| #single | `41.8% <0%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21541?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/sorting.py](https://codecov.io/gh/pandas-dev/pandas/pull/21541/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc29ydGluZy5weQ==) | `98.2% <100%> (ø)` | :arrow_up: |
| [pandas/core/arrays/categorical.py](https://codecov.io/gh/pandas-dev/pandas/pull/21541/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2NhdGVnb3JpY2FsLnB5) | `95.63% <0%> (-0.06%)` | :arrow_down: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/21541/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `96.12% <0%> (-0.01%)` | :arrow_down: |
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/21541/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.62% <0%> (ø)` | :arrow_up: |
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/21541/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `94.19% <0%> (+0.01%)` | :arrow_up: |
| [pandas/core/indexes/category.py](https://codecov.io/gh/pandas-dev/pandas/pull/21541/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9jYXRlZ29yeS5weQ==) | `97.28% <0%> (+0.18%)` | :arrow_up: |
| [pandas/core/dtypes/cast.py](https://codecov.io/gh/pandas-dev/pandas/pull/21541/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2Nhc3QucHk=) | `88.49% <0%> (+0.26%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21541?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21541?src=pr&el=footer). Last update [2625759...eebb8cf](https://codecov.io/gh/pandas-dev/pandas/pull/21541?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @jreback I've addressed all your comments the best I can.
Changelog updated, snipped used in the issue added as a test case (with minor changes to accomodate to different py3 recursion settings)
Comments to the loop and internal method added describing what is going on there as best as I understand it. Also, just to doublecheck that I've updated the correct changelog - if this is merged- it will be in 0.23.2, right?  Again :(
```
3.5 is not installed; attempting download
Downloading archive: https://s3.amazonaws.com/travis-python-archives/binaries/ubuntu/14.04/x86_64/python-3.5.tar.bz2
$ curl -sSf -o python-3.5.tar.bz2 ${archive_url}
curl: (56) SSL read: error:00000000:lib(0):func(0):reason(0), errno 104
Unable to download 3.5 archive. The archive may not exist. Please consider a different version.
``` i restarted that job. ping on green. @WillAyd I've addressed your review comments @jreback your comments are addressed as well thanks @Templarrr 

@WillAyd lgtm. pls approve & merge when satisifed Thanks @Templarrr ! Yay, I'm pandas contributor :-D 
Thanks for the reviews and comments!