[scikit-learn] always Squash and Merge?

[Andreas Mueller]

Hey.

This is a continuation of the discussion we had on squashing in June:
https://mail.python.org/pipermail/scikit-learn/2016-June/000121.html

I thought we discussed this again after the "squash and merge" feature 
was introduced, but I couldn't find the thread.

I think Joel, me and some others where recently using the github "squash 
and merge" feature,
which I think is great. It removes burden from the contributors and 
makes for a "clean" (or fake) history.
I like it because it makes cherry-picking easy and allows a pretty 
simple analysis of what's happening.

When doing some backports, I realized that some people (including Gael) 
didn't use it.

Is there a reason not to use squash and merge? Should we make it policy?

The one case where I think we might not want it is in case there are 
multiple authors in a PR.
Other than that, I don't really see a downside.

Wdyt?

Andy