CI: Check all commit messages for string (MR 6301)

This way you can add e.g. [ci:skip-vercheck] to the commit it actually
belongs to instead of having to add it to the last one in the series,
which is unintuitive. It still doesn't only run the check on commits
that contain the message, which might be a nice future improvement.

.ci/lib/common.py

@@ -41,7 +41,9 @@ def add_upstream_git_remote():
 
 
 def commit_message_has_string(needle):
-    return needle in run_git(["show", "-s", "--format=full", "HEAD"])
+    base_commit = get_base_commit()
+
+    return needle in run_git(["log", "--pretty=format:%B", f"{base_commit}..HEAD"])
 
 
 def run_pmbootstrap(parameters):