Signing commits is a local affair, you don't want to be uploading your private key to Github. You sign locally via command line and then push to Github where they get verified with your public key.
It’s even worse, if somebody rebase-merges a pull request that you authored (thereby creating a new commit that you did not author), GitHub will show you as the author (without a separate committer, like it normally does when author and committer differ), and put “verified” next to it, which usually means that they verified that it was signed by your GPG key, but in this case, it means that the commit was created by GitHub.
Well, yes. The question was whether you can sign _on GitHub_, so your private key has to be available to GitHub. You can always sign locally if you don't trust GitHub.
Well that was my point - I wonder why we haven't set up a system that lets me sign the merge commit. Otherwise it's a commit purported to be authored by me but when you look it's actually signed by someone else.
