If you consistently use feature branches and have meaningful summary commit messages in the merge commits as they land on the mainline branch what you're describing is simply:

    git log --max-parents=1
    git log --min-parents=2

E.g. try this on the git.git repository (not perfect there, since Junio doesn't use this pattern all the time), but it's good enough for a quick demo.

The most useful convention in a DAG like git is to have the topology of your commits reflect your workflow.