search: always search indexed branches if available

Created by: keegancsmith

This allows our search layer to search both indexed and unindexed revisions for a repository at the same time.

A user needing to do this is a bit of a rare case outside of version contexts. And now that we index version context branches this will only happen when explicitly opting into multiple branches. There can be some weird reporting artefacts (eg a repo being listed as both indexed and unindexed search). But better to have indexed search than not.

Fixes https://github.com/sourcegraph/sourcegraph/issues/12064