Allow magithub-edit buffers to invoke popups upon submission

[vermiculus]

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From #264.

Right now, this is a little complicated. Once a popup is invoked, control returns back to the top-level while the popup waits for user input. As this happens (i.e., immediately when magit-invoke-popup returns), magithub-edit-submit closes the buffer and tries to get every buffer back to the way it was.

We could work around this limitation by using a special return symbol in the callback to avoid all this. This will be...complicated.

@tarsius -- re transient.el, I don't expect you'll be changing this behavior from magit-popup into transient.el, but I'd thought I'd ask.

[tarsius]

That's right.

[vermiculus]

Thanks :-)