2 callbacks have been added to git_rebase_options, git_rebase_commit_signature_cb and git_rebase_commit_signature_field_cb. When git_rebase_commit_signature_cb is present in git_rebase_options, it will be called whenever git_rebase_commit is performed, giving an opportunity to sign the commit. The signing procedure can be skipped if the callback specifies passthrough as the error. The git_rebase_commit_signature_field_cb will only be called if the other callback is present or did not passthrough, and it provides means to specify which field a signature is for.
Git_rebase_options was chosen as the home for these callbacks as it keeps backwards compatibility with the current rebase api.