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.
-- 
Duy