Add script for verifying source code changes

Also use the new script in the check-pr.yml workflow.

.github/workflows/check-pr.yml

@@ -54,3 +54,15 @@ jobs:
         path: html/issue*.html
         retention-days: 7
         if-no-files-found: ignore
+
+    - name: Check source code changes
+      continue-on-error: true
+      run: |
+        git fetch --depth 2 origin master
+        if ! git diff --quiet origin/master HEAD -- src
+        then
+          if ! bin/check-html-diffs.sh origin/master always
+          then
+            echo "::warning title=html-diffs::Generated HTML files do not match"
+          fi
+        fi

bin/check-html-diffs.sh

@@ -0,0 +1,18 @@
+#!/bin/sh
+old=$(mktemp -d ./mailing.old.XXXXXX)
+new=$(mktemp -d ./mailing.new.XXXXXX)
+echo Building lists at HEAD
+make clean
+make lists -j4
+sed -i 's/Revised ....-..-.. at ..:..:.. UTC/Revised .../' mailing/*.html
+mv mailing $new
+git checkout ${1-origin/master} -- src
+echo Building lists with code from `git rev-parse origin/master`
+make clean
+make lists -j4
+git checkout HEAD -- src
+sed -i 's/Revised ....-..-.. at ..:..:.. UTC/Revised .../' mailing/*.html
+mv mailing $old
+diff -u -r --color=${2:-auto} $old $new || exit
+rm -r $old $new
+echo No changes to HTML files