Searching for `"tap(val =>" count:30` is slow on sourcegraph.com

Created by: ijt

Currently, searches like "tap(val =>" count:30 take far longer than 1s to run. In fact, this search gives a 504, probably due to exceeding the nginx timeout on the order of minutes.

However, when run with index:only it returns in 0.13 seconds.

This suggests it would be a better user experience for sourcegraph.com to default to index:only until we can track down why some of the default repos are not in the list of zoektRepos.