Add support for git duet-commit

[trayo]

git-duet uses the git duet-commit command for committing with an author and committer. We use it when two people are pairing. It would be awesome to be able to have Gitx use the same command. We tried aliasing git commit to send git duet-commit in our gitconfig, but Gitx didn't seem to respect that.

Thanks!

cc: @JordanICollier