Merge diverged 2.0 branch from git.haskell.org

for some strange reason, git.haskell.org had a different
(diverged) version of the 2.0 branch.

This merge commit unifies the histories again for the benefit
of ghc.git's gitlink in the ghc-8.2 branch, which happen to refer
to commit 3aa2d69 which had been
tagged as https://github.com/haskell/cabal/releases/tag/Cabal-v2.0.0.1
but lives in that mysterious alternative branch...

NB: this commit refers to the same tree object as the Cabal-v2.0.0.2
    tag; IOW, 'git diff '  relative to that tag is empty.

Author: hvr