Ha, they're going back to the way repo tab navigation used to work a long time ago, with the tabs on the top, like [1] and [2].

Arguably it's a nicer look with some UX benefits.

[1] - https://camo.githubusercontent.com/fec1c4ab93659e759682ad5db...

[2] - http://cdn.appappeal.com/pictures/6089/screenshot.png