Change default merging style

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@mitsuhiko I realize I've merged a few PRs on the default merge style — which is Merge Commit — could we change this to Squash & Merge? And possibly only allow that?

Sorry the branch history is now messier. I should have checked more carefully. All the repos I maintain are set to always squash so I'm not in the habit of checking.

If you get to this immediately and really care, it's maybe fine to push -f, I've made a version with linear history at https://github.com/mitsuhiko/insta/commits/c94baa4f08a107cd31d207989d10a0e429d169f5/

[max-sixty]

• I think we're too late to force push a fixed linear history, so we'll have to live with a small section of history being messy
• I'll take care to merge by squashing
• Would be good to change the default to squashing / only allow squashing