search: Repos.MinLastChanged takes into account search contexts

Created by: keegancsmith

Zoekt polls Sourcegraph to find out which repositories to index. We introduced MinLastChanged so that when polling it only needs to then check gitserver for those repos which have changed since a certain time. However, part of what could change an indexed repos is it being included in a search context. As such we updated MinLastChanged to also include any repository in a search context which has changed.

We confirm that updated_at is correctly updated anytime the set of repositories changes.

Note: this is just a heuristic, zoekt will still occasionally poll everything and not use MinLastChanged.