maintainers/scripts/haskell: unify pkg set update into single commit

Currently, every package set consists of three commits, generated by
update-hackage.sh, update-stackage.sh and
regenerate-hackage-packages.sh, respectively. This is suboptimal, as it
necessarly causes intermediate states of Nixpkgs where the generated
hackage-packages.nix and all-cabal-hasehs and/or the hackage2nix
configuration files are out of sync. Ideally, running
regenerate-hackage-packages.sh is a no-op for every Nixpkgs revision.

This is achieved by adding a wrapper script, update-package-set.sh,
which runs the individual moving parts and commits the result.

maintainers/scripts/haskell/merge-and-open-pr.sh

@@ -80,13 +80,7 @@ echo "Merging https://github.com/NixOS/nixpkgs/pull/${curr_haskell_updates_pr_nu
 gh pr merge --repo NixOS/nixpkgs --merge "$curr_haskell_updates_pr_num"
 
 # Update stackage, Hackage hashes, and regenerate Haskell package set
-echo "Updating Stackage..."
+./maintainers/scripts/haskell/update-package-set.sh
-./maintainers/scripts/haskell/update-stackage.sh --do-commit
-echo "Updating Hackage hashes..."
-./maintainers/scripts/haskell/update-hackage.sh --do-commit
-echo "Regenerating Hackage packages..."
-# Using fast here because after the hackage-update eval errors will likely break the transitive dependencies check.
-./maintainers/scripts/haskell/regenerate-hackage-packages.sh --fast --do-commit
 
 # Push these new commits to the haskell-updates branch
 echo "Pushing commits just created to the remote $push_remote/haskell-updates branch..."

maintainers/scripts/haskell/update-package-set.sh

@@ -0,0 +1,53 @@
+#! /usr/bin/env nix-shell
+#! nix-shell -i bash
+#! nix-shell -p git -I nixpkgs=.
+set -euo pipefail
+
+filesToStage=(
+  'pkgs/data/misc/hackage/pin.json'
+  'pkgs/development/haskell-modules/configuration-hackage2nix/stackage.yaml'
+  'pkgs/development/haskell-modules/hackage-packages.nix'
+)
+
+if ! git diff --quiet --cached; then
+  echo "Please commit staged changes before running $0" >&2
+  exit 100
+fi
+
+if ! git diff --quiet -- "${filesToStage[@]}"; then
+  echo -n "Please commit your changes to the following files before running $0: " >&2
+  echo    "${filesToStage[@]}" >&2
+  exit 100
+fi
+
+stackage_diff="$(./maintainers/scripts/haskell/update-stackage.sh)"
+hackage_diff="$(./maintainers/scripts/haskell/update-hackage.sh)"
+readonly stackage_diff hackage_diff
+
+# Prefer Stackage version diff in the commit header, fall back to Hackage
+if [[ -n "$stackage_diff" ]]; then
+  commit_message="haskellPackages: stackage $stackage_diff"
+  if [[ -n "$hackage_diff" ]]; then
+    commit_message="$commit_message
+
+all-cabal-hashes: $hackage_diff"
+  fi
+elif [[ -n "$hackage_diff" ]]; then
+  commit_message="haskellPackages: hackage $hackage_diff
+
+all-cabal-hashes: $hackage_diff"
+else
+  echo "Neither Hackage nor Stackage changed. Nothing to do." >&2
+  exit 0
+fi
+
+commit_message="$commit_message
+
+(generated by maintainers/scripts/haskell/update-package-set.sh)"
+
+# Using fast here because after the hackage-update eval errors will likely break the transitive dependencies check.
+./maintainers/scripts/haskell/regenerate-hackage-packages.sh --fast
+
+# A --do-commit flag probably doesn't make much sense
+git add -- "${filesToStage[@]}"
+git commit -m "$commit_message"