Detect "ignored yet tracked" files in GitLab CI

Ensure that no .gitignore file in the tree contains patterns matching
files tracked by Git by adding a simple script that detects such
situations and running that script as part of the "misc" job in the
"precheck" stage of every GitLab CI pipeline.

.gitlab-ci.yml

@@ -443,6 +443,7 @@ misc:
     - test ! -f CHANGES.SE || sh util/check-line-length.sh CHANGES.SE
     - rm CHANGES.tmp
     - sh util/check-categories.sh
+    - sh util/check-gitignore.sh
     - if git grep SYSTEMTESTTOP -- ':!.gitlab-ci.yml'; then echo 'Please use relative paths instead of $SYSTEMTESTTOP.'; exit 1; fi
     - bash util/unused-headers.sh
     - bash util/xmllint-html.sh

util/check-gitignore.sh

@@ -0,0 +1,23 @@
+#!/bin/sh
+
+# Copyright (C) Internet Systems Consortium, Inc. ("ISC")
+#
+# SPDX-License-Identifier: MPL-2.0
+#
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0.  If a copy of the MPL was not distributed with this
+# file, you can obtain one at https://mozilla.org/MPL/2.0/.
+#
+# See the COPYRIGHT file distributed with this work for additional
+# information regarding copyright ownership.
+
+ignored_yet_tracked="$(git ls-files --cached --ignored --exclude-standard | git check-ignore --verbose --stdin --no-index)"
+
+if [ -n "${ignored_yet_tracked}" ]; then
+	echo "The following .gitignore files contain patterns matching tracked files:"
+	echo
+	echo "${ignored_yet_tracked}"
+	echo
+	echo "Please adjust the contents of the above .gitignore files and/or the names of the tracked files."
+	exit 1
+fi