core: Add ability for repositories to have a "parent"

This will be useful for ostbuild; a user can create their own archive
mode repository which transparently inherits objects from the
root-owned one in /ostree.

src/libostree/ostree-repo.h

@@ -92,14 +92,12 @@ gboolean      ostree_repo_abort_transaction (OstreeRepo     *self,
                                              GCancellable   *cancellable,
                                              GError        **error);
 
-gboolean      ostree_repo_find_object (OstreeRepo           *self,
-                                       OstreeObjectType      objtype,
-                                       const char           *checksum,
-                                       GFile               **out_stored_path,
-                                       char                **out_pack_checksum,
-                                       guint64              *out_pack_offset,
-                                       GCancellable         *cancellable,
-                                       GError              **error);
+gboolean      ostree_repo_has_object (OstreeRepo           *self,
+                                      OstreeObjectType      objtype,
+                                      const char           *checksum,
+                                      gboolean             *out_have_object,
+                                      GCancellable         *cancellable,
+                                      GError              **error);
 
 gboolean      ostree_repo_stage_object (OstreeRepo       *self,
                                         OstreeObjectType  objtype,