Update from `magit-popup` to `transient`

[matthew-piziak]

Just wondering what you think of this. magit now uses transient instead of magit-popup. The migration is fairly prosaic. There are backward compatibility problems of course, but magit has gone for it.

[wbolster]

yeah slowly doing the same for my init.el, i guess it's the way forward even though the benefits are small or non existent for this project currently