repo: Tweak traversal API

It's convenient for bindings if we have a version that doesn't mutate
the hash table, because they pass temporary hash tables as input.

src/libostree/ostree-repo.h

@@ -414,19 +414,20 @@ gboolean ostree_repo_list_objects (OstreeRepo                  *self,
 
 GHashTable *ostree_repo_traverse_new_reachable (void);
 
-gboolean ostree_repo_traverse_dirtree (OstreeRepo         *repo,
-                                       const char         *commit_checksum,
-                                       GHashTable         *inout_reachable,
-                                       GCancellable       *cancellable,
-                                       GError            **error);
-
 gboolean ostree_repo_traverse_commit (OstreeRepo         *repo,
                                       const char         *commit_checksum,
                                       int                 maxdepth,
-                                      GHashTable         *inout_reachable,
+                                      GHashTable        **out_reachable,
                                       GCancellable       *cancellable,
                                       GError            **error);
 
+gboolean ostree_repo_traverse_commit_union (OstreeRepo         *repo,
+                                            const char         *commit_checksum,
+                                            int                 maxdepth,
+                                            GHashTable         *inout_reachable,
+                                            GCancellable       *cancellable,
+                                            GError            **error);
+
 /**
  * OstreeRepoPruneFlags:
  * @OSTREE_REPO_PRUNE_FLAGS_NONE: No special options for pruning