netlify: try to fix dirty build detection

ci/www/changed.sh

@@ -0,0 +1,23 @@
+#!/usr/bin/env bash
+
+# Copyright 2019 Materialize, Inc. All rights reserved.
+#
+# This file is part of Materialize. Materialize may not be used or
+# distributed without the express permission of Materialize, Inc.
+#
+# changed.sh — determines whether Netlify should rebuild the website.
+
+set -euo pipefail
+
+cd "$(dirname "$0")/../.."
+
+branch=$(git rev-parse --abbrev-ref HEAD)
+if [[ "$branch" = master ]]; then
+    spec=HEAD^
+else
+    spec=master...
+fi
+
+# Netlify doesn't follow symlinks as of 03 Feb 2020, so we need to explicitly
+# check for changes in doc/user too.
+exec git diff --quiet "$spec" -- doc/user www

netlify.toml

@@ -2,6 +2,7 @@
 base = "www"
 publish = "public"
 command = "hugo --gc --baseURL $URL"
+ignore = "../ci/www/changed.sh"
 environment = { HUGO_VERSION = "0.58.3" }
 
 [context.deploy-preview]