CI: check for changes to autogenerated files

.github/workflows/autogenerated-files.yml

@@ -0,0 +1,46 @@
+name: Autogenerated files check
+
+on:
+  pull_request:
+    paths:
+      - .github/workflows/autogenerated-files.yml
+      - README.md
+      - completions/**
+      - docs/Manpage.md
+      - manpages/brew.1
+
+permissions:
+  contents: read
+
+env:
+  HOMEBREW_DEVELOPER: 1
+  HOMEBREW_NO_AUTO_UPDATE: 1
+
+jobs:
+  autogenerated:
+    runs-on: ubuntu-22.04
+    if: github.repository == 'Homebrew/brew'
+    steps:
+      - name: Set up Homebrew
+        id: set-up-homebrew
+        uses: Homebrew/actions/setup-homebrew@master
+
+      - name: Cache Bundler RubyGems
+        uses: actions/cache@v1
+        with:
+          path: ${{ steps.set-up-homebrew.outputs.gems-path }}
+          key: ${{ runner.os }}-rubygems-${{ steps.set-up-homebrew.outputs.gems-hash }}
+          restore-keys: ${{ runner.os }}-rubygems-
+
+      - name: Check for changes to autogenerated files
+        id: check
+        run: |
+          if brew generate-man-completions
+          then
+            echo "This PR modifies autogenerated files!" >&2
+            echo "Please ensure their source files are updated and then run the following:
+              brew generate-man-completions" >&2
+            exit 1
+          else
+            exit 0
+          fi