Ensure Zoekt index is updated when default branch changes

When a repo is updated we just tell Zoekt to fetch updates to the clone. But Zoekt (by default) just indexes whatever is set as HEAD in it's local bare repo copy. It's likely this does not change when the default branch in GitLab changes.

Reading https://stackoverflow.com/questions/29296232/why-does-bare-repository-have-head suggests there may be a way to force the local bare repo to change the value of HEAD. This may require adding to the Zoekt API to pass in a flag (or branch name) to ensure that this is updated.