search: specify branch for zoektSearchHEADOnlyFiles
Created by: keegancsmith
Structural search expects to only search HEAD (currently). However, it was using a RepoSet which will return results for any branch we have indexed. I believe this would result in potential benign false positives for files to search. We switch to instead passing in the computed RepoBranches (which should be limited to HEAD).
Part of https://github.com/sourcegraph/sourcegraph/issues/11902