[DEVOPS-936] nix: Don't use git for stylishHaskell diff
[DEVOPS-936] nix: Don't use git for stylishHaskell diff

Use plain old diff because git diff records file mode changes which
produces warnings when the patch is applied.

input-output-hk/cardano-sl,input-output-hk/pos-haskell-prototype,input-output-hk/cardano-sl,input-output-hk/cardano-sl,input-output-hk/pos-haskell-prototype,input-output-hk/pos-haskell-prototype,input-output-hk/cardano-sl