Change Github TABSIZE to 4 and make code review easier

Have you ever seen weird indentations when doing code review? Mostly, the cause is that GitHub's default TABSIZE setting is 8 instead of 4 while some old code uses TAB for indentation. Fortunately, we can easily fix it by change this setting. 8 is not an ideal default TABSIZE nowadays, right? so it is all GitHub's fault!

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