core: Allow adding related objects to commits

This will be used to store the set of components.

src/libostree/ostree-repo.h

@@ -245,6 +245,7 @@ gboolean      ostree_repo_stage_commit (OstreeRepo   *self,
                                         const char   *subject,
                                         const char   *body,
                                         GVariant     *metadata,
+                                        GVariant     *related_objects,
                                         const char   *content_checksum,
                                         const char   *metadata_checksum,
                                         char        **out_commit,