No way to start a discussion on commit messages.
As a newcomer project, Music sees it's fair share of first time contributors with the weirdest commit messages (eg. GNOME/gnome-music!26 (closed)). We always direct them to https://wiki.gnome.org/Git/CommitMessages and in bugzilla reviews I usually add a short blurb about what I would like to see there.
In gitlab there is no way to make a discussion point of a commit message, like there is for code. To me commit messages are an important part of the commit, so this is sorely missing.
I found this issue on the gitlab issue tracker: https://gitlab.com/gitlab-org/gitlab-ce/issues/38602 . Maybe we can make a point of this as desired feature in the gnome gitlab instance.