commit-graph: validate layers for generation data

We need to be extra careful that we don't use corrected
commit dates from any layer of a commit-graph chain if there is a
single commit-graph file that is missing the generation_data chunk.
Update validate_mixed_generation_chain() to correctly update each
layer to ignore the generation_data chunk in this case. It now also
returns 1 if all layers have a generation_data chunk. This return
value will be used in the next change.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Reviewed-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit-graph.c

@@ -614,19 +614,29 @@ static struct commit_graph *load_commit_graph_chain(struct repository *r,
 	return graph_chain;
 }
 
-static void validate_mixed_generation_chain(struct commit_graph *g)
+/*
+ * returns 1 if and only if all graphs in the chain have
+ * corrected commit dates stored in the generation_data chunk.
+ */
+static int validate_mixed_generation_chain(struct commit_graph *g)
 {
-	int read_generation_data;
+	int read_generation_data = 1;
+	struct commit_graph *p = g;
 
-	if (!g)
-		return;
+	while (read_generation_data && p) {
+		read_generation_data = p->read_generation_data;
+		p = p->base_graph;
+	}
 
-	read_generation_data = !!g->chunk_generation_data;
+	if (read_generation_data)
+		return 1;
 
 	while (g) {
-		g->read_generation_data = read_generation_data;
+		g->read_generation_data = 0;
 		g = g->base_graph;
 	}
+
+	return 0;
 }
 
 struct commit_graph *read_commit_graph_one(struct repository *r,