Branch/commit/tag selector dropdown is slow on giant monorepo (100K+ branches)

Created by: dadlerj

Reported by https://app.hubspot.com/contacts/2762526/company/554275594

Accessing non-default branches does appear to work, but the branches list is very slow to load

For context, this is a >10GB repository with 100K+ branches

I'm not sure if the user was referring to the branch dropdown or the branches page (e.g. https://sourcegraph.com/github.com/sourcegraph/sourcegraph/-/branches). They seem to use different GraphQL endpoints: https://sourcegraph.com/.api/graphql?RepositoryGitRefs and https://sourcegraph.com/.api/graphql?RepositoryGitBranchesOverview

The solution here might be along the same lines as what @lguychard did in https://github.com/sourcegraph/sourcegraph/pull/8515 -- just improve the initial/naive case perf rather than trying to optimize the backend.

CC @keegancsmith