simplify cabal-fmt.nix and update docs (#515)

CONTRIBUTING.adoc

@@ -30,6 +30,7 @@ NOTE: You may want to consider using https://github.com/target/lorri[lorri] as a
 The shell comes with some tools for fixing various simple problems that the CI will complain about. Specifically:
 
 - `fix-stylish-haskell` will re-format all the Haskell sources correctly.
+- `fix-cabal-fmt` will re-format all the Cabal sources correctly.
 - `fix-purs-tidy` will re-format all the Purescript sources correctly.
 - `fix-png-optimization` will optimize all PNGs in the repository.
 

nix/tests/cabal-fmt.nix

@@ -1,15 +1,6 @@
 { runCommand, fixCabalFmt, src, lib, diffutils, glibcLocales }:
 let
-  src' = lib.cleanSourceWith {
-    inherit src;
-    filter = with lib;
-      name: type:
-        let baseName = baseNameOf (toString name); in
-        (
-          (type == "regular" && hasSuffix ".cabal" baseName) ||
-          (type == "directory" && (baseName != "dist-newstyle" && baseName != "dist" && baseName != ".stack-work"))
-        );
-  };
+  src' = lib.sourceFilesBySuffices src [ ".cabal" ];
 in
 runCommand "cabal-fmt"
 {
@@ -30,7 +21,7 @@ runCommand "cabal-fmt"
     diff -ur orig cabal > $out/cabal.diff
     echo "file none $out/cabal.diff" > $out/nix-support/hydra-build-products
     echo "*** cabal-fmt found changes that need addressed first"
-    echo "*** Please run \`fix-stylish-haskell\` and commit changes"
+    echo "*** Please run \`fix-cabal-fmt\` and commit changes"
     echo "*** or apply the diff generated by hydra if you don't have nix."
     exit $EXIT_CODE
   else