gomod: update zoekt to support query.RepoBranches
Created by: keegancsmith
We will be using query.RepoBranches to query zoekt once we support multiple branches. Below is the commit message from zoekt:
RepoBranches is like our RepoSet query atom. Except it allows us to specify which branches to search in each repository in the set. This is required for Sourcegraph to efficiently construct queries which search across different branches. See RFC 150 Version Contexts for more context.