fuzz-commit-graph: properly free graph struct

Use the provided free_commit_graph() to properly free the commit graph
in fuzz-commit-graph. Otherwise, the fuzzer itself leaks memory when the
struct contains pointers to allocated memory.

Signed-off-by: Josh Steadmon <steadmon@google.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

fuzz-commit-graph.c

@@ -12,7 +12,7 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size)
 	initialize_the_repository();
 	g = parse_commit_graph((void *)data, size);
 	repo_clear(the_repository);
-	free(g);
+	free_commit_graph(g);
 
 	return 0;
 }