Find a topic branch containing a commit

Before I start doing anything silly because I don't know it can already be done without waving my C wand like a mad man...

I often do this: find a commit of interest, the commit itself is not enough so I need a full patch series to figure out what's going, so I fire up "git log --graph --oneline" and manually search that commit and trace back to the merge point, then I can "git log --patch". Is there an automatic way to accomplish that? Something like "git branch --contains" (or "git merge --contains")?

PS. Sometimes I wish we could optionally save cover letter in the merge commit. Sometimes the "big plan" is hard to see by reading individual commit messages.