ci: Blacklist .c and .h in the commit message subject prefix

`boxes.c: Do that` should be just `boxes: Do that`.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/519

.gitlab-ci/check-commit-log.sh

@@ -32,6 +32,11 @@ function commit_message_subject_is_compliant() {
     return 1
   fi
 
+  if echo "$commit_message_subject" | grep -qe "\.[ch]:"; then
+    echo " - message subject prefix should not include .c, .h, etc."
+    return 1
+  fi
+
   return 0
 }