indexed-search: use zoekt.FileMatch.Version for SHA
Created by: keegancsmith
A while ago we added "IndexedHEADCommit" for finding the sha for a branch we searched in zoekt. However, the Zoekt API returns the SHA in the result struct. It will also be the actual sha, and won't suffer from potential race conditions. We also get the benefit of a smaller struct for RepositoryRevisions.
This change is additionally required once we have multiple branches indexed since we will need a way to resolve the commit for more than just HEAD.