plumbing: support commits extra headers, support jujutsu signed commit [5.x]#1633

pjbgf merged 1 commit intogo-git:releases/v5.xgo-git/go-git:releases/v5.xfrom

baloo:baloo/release-5.x/jj-signed-commitsbaloo/go-git:baloo/release-5.x/jj-signed-commitsCopy head branch name to clipboard