core: Add --tar-autocreate-parents option for commit

The tar files we're making of artifacts don't include parent
directories.  Now we could change the builder to make them, but we can
also just autocreate them on import.  Mode 0755 with no xattrs seems
OK here.

src/libostree/ostree-mutable-tree.h

@@ -77,6 +77,13 @@ gboolean ostree_mutable_tree_lookup (OstreeMutableTree   *self,
                                      OstreeMutableTree  **out_subdir,
                                      GError             **error);
 
+gboolean
+ostree_mutable_tree_ensure_parent_dirs (OstreeMutableTree  *self,
+                                        GPtrArray          *split_path,
+                                        const char         *metadata_checksum,
+                                        OstreeMutableTree **out_parent,
+                                        GError            **error);
+
 gboolean ostree_mutable_tree_walk (OstreeMutableTree   *self,
                                    GPtrArray           *split_path,
                                    guint                start,