Failing config related test

```c
void test_config_write__can_set_a_value_to_NULL(void)
{
	git_repository *repository;
	git_config *config;

	repository = cl_git_sandbox_init("testrepo.git");

	cl_git_pass(git_repository_config(&config, repository));
	cl_git_pass(git_config_set_string(config, "a.b.c", NULL));
	git_config_free(config);

	cl_git_sandbox_cleanup();
}
```

Fails with message `"Race condition when writing a config file (a cvar has been removed)"`

/cc @carlosmn