Porcelain/Plumbing

Heya,

During my efforts gsoc efforts, I stumbled across (or actually, was pointed at) 'git branch --contains' to list only branches that contain a specific commit. The problem here is that 'git branch' is listed as porcelain, and that there seems to be no plumbing equivalent of this feature. I have the same problem with 'git log -S', although 'git log' is pretty sturdy output wise (if you specify the output with '--pretty=format:' that is), the idea is the same. My question is, is it intentional that there is no plumbing equivalent forsome functionality, and, in such cases, is it 'ok' to use the porcelain from a script? (I think it is not.) I've been recommended to stick with the plumbing as much as possible from my code, but with the above two cases I've been unsure on what to do. What is the advised course of action here?