search: return branches to index to Zoekt

Created by: keegancsmith

If we have version contexts enabled we will tell Zoekt to index the branches in the version context. There is a related Zoekt change to pass in the repository as well as understand the Branches field being set.

Previously Zoekt was responsible for resolving HEAD. Now that we will do more than HEAD, it was easier to implement the resolving logic in Sourcegraph. This will both simplify the responsiblilities of zoekt-sourcegraph-indexserver as well as allow us to optimize patterns around resolving version context revisions.

Supersedes https://github.com/sourcegraph/sourcegraph/pull/10851 Part of #6728