overriding/removing inherited credential.helper

In credential.c, line 67:

    if (!strcmp(key, "helper"))
        string_list_append(&c->helpers, value);

In global config, I add one credential helper.
But I do not want to use any credential helper in a specific repository.
Currently there is no way in local config to override and remove
inherited credential helper.
Of course, I do not want to change global config.

Could you suggest a patch?