Update cabal-hashes and add script to ease future updates

Thanks to vaibhavsagar for the update script [1].

[1] https://github.com/vaibhavsagar/experiments/blob/f3f807212b82f07485c3df041cd8d6236ecb94e2/bump-all-cabal-hashes/updater

all-cabal-hashes.nix

-let rev = "e8e76bc26a994aee313e571dc4e6701398d17a42";
-in
-{
-  url    = "https://github.com/commercialhaskell/all-cabal-hashes/archive/${rev}.tar.gz";
-  sha256 = "16rnyxqmr93ahml0fjfa6hmjpmx8sbpfdr52krd2sd6ic9n5p5ix";
-}

default.nix

@@ -28,7 +28,15 @@ let
   };
 
   overrides = self: super: rec {
-    all-cabal-hashes = self.fetchurl (import ./all-cabal-hashes.nix);
+    # Use scripts/update-cabal-hashes.sh to update this
+    all-cabal-hashes =
+      let
+        versions = builtins.fromJSON (builtins.readFile ./scripts/cabal-hashes.json);
+        fetch = { owner, repo, rev, sha256, ... }: self.fetchurl {
+            inherit sha256;
+            url = "https://github.com/${owner}/${repo}/archive/${rev}.tar.gz";
+        };
+      in fetch versions;
 
     # Should this be self?
     ghcHEAD = ghc super;

scripts/cabal-hashes.json

+{
+  "owner": "commercialhaskell",
+  "repo": "all-cabal-hashes",
+  "branch": "hackage",
+  "message": "Update from Hackage at 2019-07-29T18:23:42Z",
+  "rev": "8cb9e194879225aab2a35838b4ab4b53f85d8eb8",
+  "sha256": "1krylhakvipxy2yiiswy2jq0f1giczx3y9gy335kq5r84brpar3v"
+}

scripts/update-cabal-hashes.sh

+#! /usr/bin/env nix-shell
+#! nix-shell -i bash
+#! nix-shell -p curl jq nix
+
+set -eufo pipefail
+
+FILE=scripts/cabal-hashes.json
+
+OWNER=$(jq -r '.owner' < "$FILE")
+REPO=$(jq -r '.repo' < "$FILE")
+BRANCH=$(jq -r '.branch' < "$FILE")
+
+JSON=$(curl "https://api.github.com/repos/$OWNER/$REPO/branches/$BRANCH")
+REV=$(echo "$JSON" | jq -r '.commit.sha')
+MESSAGE=$(echo "$JSON" | jq -r '.commit.commit.message')
+SHA256=$(nix-prefetch-url "https://github.com/$OWNER/$REPO/archive/$REV.tar.gz")
+TJQ=$(jq '{owner: $owner, repo: $repo, branch: $branch, message: $message, rev: $rev, sha256: $sha256}' \
+  --arg owner "$OWNER" \
+  --arg repo "$REPO" \
+  --arg branch "$BRANCH" \
+  --arg message "$MESSAGE" \
+  --arg rev "$REV" \
+  --arg sha256 "$SHA256" \
+  < "$FILE")
+[[ $? == 0 ]] && echo "${TJQ}" >| "$FILE"