search: only search requested indexed branches

Created by: keegancsmith

Previously our indexed search code was updated to detect which branches are indexed, and if indexed search those repositories. However, we hadn't yet added code which told Zoekt which branches to search.

This commit records the mapping between the Zoekt branch and the revision specified by the user. This allows us to tell Zoekt which branches to search as well as map back Zoekt results to the user specified revision.

Historical Context: Technically it would be more correct to link to the SHA and easier to implement. However, that would "lose" your current browsing context.

Part of #6728