Proposal: accept pull requests on GitHub

[Nikita Karetnikov]

> I would recommend against moving code reviews to Github.
> I like it and use it all the time for my own projects, but for a large
> project like GHC, its code reviews are too basic (comments get lost in
> multi-round reviews), and its customisation an process enforcement is
> too weak; but that has all been mentioned already on the
> https://ghc.haskell.org/trac/ghc/wiki/WhyNotGitHub page you linked.

At least you're able to submit comments!  I just had my second
interaction with arc/phab and I have to say that I really hate both now.
The former is not flexible enough and lacks documentation, the latter is
just plain confusing.

I was trying to update an existing phab diff (D1334), but I had no idea
what would be submitted, and I'm still not sure whether I did that okay
or not.  Oh, slyfox tells me that I overwrote my previous changes, nice!
In the process, I also created a new revision by mistake.  The web UI
didn't help either since there's so much stuff: diffs, revisions, ids.
Is it okay to have multiple diffs in a single phab differential after
updating?  No idea.

After that I was struggling to reply to rwbarton.  I hit "Done" and
added my comment, but both things were marked as "Unsubmitted" (or
something).  After a while I decided to click on the button at the
bottom of the page.  Looks like it did the trick, but I have no idea
whether it's the right way or not.

Not that I'm saying that GitHub is perfect, but at least it works
instead of messing up with the work I carefully tested.