pint git commit message from single commit

gistfile1

It's not "plumbing", but it'll do exactly what you want:

$ git log --format=%B -n 1 <commit>
If you absolutely need a "plumbing" command (not sure why that's a requirement), you can use rev-list:

$ git rev-list --format=%B --max-count=1 <commit>
Although rev-list will also print out the commit sha (on the first line) in addition to the commit message.