bpo-44035: Show git diff after autoreconf and regen (GH-30117)

Author: tiran

.github/workflows/build.yml

@@ -90,8 +90,10 @@ jobs:
           if test -n "$changes"; then
             echo "Generated files not up to date."
             echo "Perhaps you forgot to run make regen-all or build.bat --regen. ;)"
-            echo "configure files must be regenerated with a specific, unpatched version of autoconf."
+            echo "configure files must be regenerated with a specific version of autoconf."
             echo "$changes"
+            echo ""
+            git diff --staged || true
             exit 1
           fi
       - name: Check exported libpython symbols