repo name searcher does serial repohasfile filtering

Created by: keegancsmith

For queries such as r:sourcegraph repoHasFile:Dockerfile the repoHasFile filtering is done serially. This should be batched.

https://github.com/sourcegraph/sourcegraph/blob/2879ccb/cmd/frontend/graphqlbackend/search_repositories.go#L60

Proposal: Adjust repoShouldBeAdded to take a slice of repositoryRevisions.