Merge branch 'tb/commit-graph-harden'

The code to parse and use the commit-graph file has been made more
robust against corrupted input.

* tb/commit-graph-harden:
  commit-graph.c: handle corrupt/missing trees
  commit-graph.c: handle commit parsing errors
  t/t5318: introduce failing 'git commit-graph write' tests

commit.c

@@ -358,7 +358,8 @@ struct tree *repo_get_commit_tree(struct repository *r,
 
 struct object_id *get_commit_tree_oid(const struct commit *commit)
 {
-	return &get_commit_tree(commit)->object.oid;
+	struct tree *tree = get_commit_tree(commit);
+	return tree ? &tree->object.oid : NULL;
 }
 
 void release_commit_memory(struct parsed_object_pool *pool, struct commit *c)