Add arcconfig, so arc diff works with phabricator.haskell.org.
Add arcconfig, so arc diff works with phabricator.haskell.org.

Signed-off-by: Edward Z. Yang <dbd597f5635f432486c5d365e9bb585b3eaa1853@cs.stanford.edu>

enolan/cabal,randen/cabal,lukexi/cabal,sopvop/cabal,Peaker/cabal,bennofs/cabal,headprogrammingczar/cabal,trskop/cabal,martinvlk/cabal,Heather/cabal,sopvop/cabal,garetxe/cabal,garetxe/cabal,sopvop/cabal,beni55/cabal,mydaum/cabal,mmakowski/cabal,kolmodin/cabal,mmakowski/cabal,martinvlk/cabal,headprogrammingczar/cabal,mydaum/cabal,thoughtpolice/cabal,bennofs/cabal,edsko/cabal,Peaker/cabal,beni55/cabal,thoughtpolice/cabal,ghc-android/cabal,enolan/cabal,kolmodin/cabal,gbaz/cabal,thomie/cabal,lukexi/cabal,edsko/cabal,thomie/cabal,themoritz/cabal,Heather/cabal,randen/cabal,themoritz/cabal,gbaz/cabal,ghc-android/cabal,trskop/cabal,themoritz/cabal