commit-graph: propagate topo_levels slab to all chain layers

The topo_levels slab is only propagated to the topmost graph
layer instead of all layers in the chain.  Commits from lower
layers appear to have no generation numbers, so the DFS
re-walks the entire ancestry.

Fix by making topo_levels visible to all layers, not just
the first one.

Signed-off-by: Kristofer Karlsson <krka@spotify.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: spkrka

commit-graph.c

@@ -2610,7 +2610,7 @@ int write_commit_graph(struct odb_source *source,
 
 	g = prepare_commit_graph(ctx.r);
 	for (struct commit_graph *chain = g; chain; chain = chain->base_graph)
-		g->topo_levels = &topo_levels;
+		chain->topo_levels = &topo_levels;
 
 	if (flags & COMMIT_GRAPH_WRITE_BLOOM_FILTERS)
 		ctx.changed_paths = 1;

t/t5324-split-commit-graph.sh

@@ -718,7 +718,7 @@ test_expect_success 'write generation data chunk when commit-graph chain is repl
 	)
 '
 
-test_expect_failure 'incremental write reads topo levels from all layers' '
+test_expect_success 'incremental write reads topo levels from all layers' '
 	git init topo-from-lower &&
 	(
 		cd topo-from-lower &&