Add image diffs against previous build to CI results

Makefile

@@ -42,6 +42,7 @@ check:
 	./check-num-pages.sh handout-beginner.pdf 1
 	./check-num-pages.sh handout-intermediate.pdf 1
 	./check-links.py cheatsheets.pdf
+	./check-diffs.py
 
 .PHONY: docs
 docs:

check-diffs.py

@@ -0,0 +1,25 @@
+#!/usr/bin/env python
+import os
+import subprocess
+import sys
+from pathlib import Path
+
+
+ROOT_DIR = Path(__file__).parent
+
+if os.environ.get('GITHUB_ACTION', '') == '':
+    print('Not running when not in GitHub Actions.')
+    sys.exit()
+
+gh_pages = ROOT_DIR.parent / 'pages'
+subprocess.run(['git', 'fetch', 'origin', 'gh-pages'], check=True)
+subprocess.run(['git', 'worktree', 'add', gh_pages, 'gh-pages'], check=True)
+
+for original in gh_pages.glob('*.png'):
+    subprocess.run(['compare',
+                    original,
+                    ROOT_DIR / 'docs/_build/html' / original.name,
+                    ROOT_DIR / 'docs/_build/html' / f'{original.stem}-diff.png'],
+                   check=True)
+
+subprocess.run(['git', 'worktree', 'remove', gh_pages])