hybrid zoekt + searcher prototype

Created by: keegancsmith

This issue is for tracking our work on a prototype to do hybrid unindexed and indexed searches. The idea is when searching a commit, you can ask searcher to only search the files that have changed since the indexed commit which should be a small set. Then you can ask zoekt to search everything else.

The hope is not only do we avoid the slow cache fill path, but for truly large repos (many gb) we have comparable perf to indexed search.

The scope of the prototype is to only work on text search. This prototype will hopefully then be able to be quickly improved to ship to customers.