Support --signoff in commit messages

[infotexture]

I regularly contribute to open-source projects that require the Git --signoff option to indicate that the commit meets the standards of the Developer Certificate of Origin (DCO).

When I push changes with git subrepo push, the commit message does not include the --signoff option, so it fails DCO checks in CI.

I'm aware of the --message option, and can use it to adjust the default commit message, but I'm not sure how to add a multiline message with that option to ensure that the signoff trailer appears on its own line at the end of the commit log message.

It would be convenient if git-subrepo would accept the --signoff option, which would allow it to add the same trailer as added by git commit --signoff.

[admorgan]

I agree this would be a good enhancement.