CI: Force changelog updates

Changelog updates are now required in MRs by default. If there is a
match for "\<skip.\?changelog\>" in the commit message, this will be
ignored.

$CI_COMMIT_MESSAGE mangles newlines so it's not possible to require the
string to be on its own line, that means it would also trigger within a
wall of text, which makes it less obvious.

Also, I wasn't able to find a CI variable which has the ref of the
branch the MR is set against, to build a tree-ish than spans over the
whole MR and not just HEAD.

Signed-off-by: Maxime “pep” Buquet <pep@bouah.net>

.forgejo/workflows/check-changelog.py

@@ -0,0 +1,40 @@
+#!/usr/bin/env python3
+# -*- encoding: utf-8 -*-
+# vim: set et ts=4 sts=4 sw=4
+
+import os
+import re
+import subprocess
+import sys
+
+
+if __name__ == '__main__':
+    # Skip this check if instructed
+    if re.search(r'skip.?changelog', os.environ['CI_COMMIT_MESSAGE'], flags=re.I) is not None:
+        print('Changelog skipped.')
+        sys.exit(0)
+
+    treeish = 'main..'
+    if os.environ['CI_PIPELINE_SOURCE'] == 'merge_request_event':
+        treeish = '{}..'.format(os.environ['CI_MERGE_REQUEST_DIFF_BASE_SHA'])
+
+    diff=subprocess.run(
+        ['git', 'diff-tree', '--no-commit-id', '-r', treeish, '--'],
+        capture_output=True,
+    )
+
+    if diff.stderr:
+        print(f'An error occured: {diff.stderr}')
+        sys.exit(1)
+
+    print(f'Files in {treeish}:\n{diff.stdout}')
+
+    # Verify if at least a single changelog file has been added or edited
+    if re.search(rb'[AM]\s+\S*changelog', diff.stdout, flags=re.I) is None:
+        print(
+            'Please update the Changelog. Use "skip-changelog" '
+            'in the commit message to skip this check.',
+        )
+        sys.exit(1)
+
+    print('Changelog has been updated as expected.')