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-prune.c

@@ -152,7 +152,7 @@ ostree_repo_prune (OstreeRepo        *self,
         {
           const char *checksum = value;
           
-          if (!ostree_repo_traverse_commit (self, checksum, depth, data.reachable,
+          if (!ostree_repo_traverse_commit_union (self, checksum, depth, data.reachable,
                                             cancellable, error))
             goto out;
         }
@@ -176,8 +176,8 @@ ostree_repo_prune (OstreeRepo        *self,
           if (objtype != OSTREE_OBJECT_TYPE_COMMIT)
             continue;
           
-          if (!ostree_repo_traverse_commit (self, checksum, depth, data.reachable,
-                                            cancellable, error))
+          if (!ostree_repo_traverse_commit_union (self, checksum, depth, data.reachable,
+                                                  cancellable, error))
             goto out;
         }
     }