disabled squashing commits

Hey all,

In what I hope is an April Fools day joke github has add a 'merge by
squashing' option to the merge button (and turned it on by default) [1].

I have taken the liberty of disabling it for the main matplotlib repository
(and will likely do the rest) in the case that it is not.


[1] https://github.com/blog/2141-squash-your-commits
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