zoekt-sourcegraph-indexserver: Use tarball redirect

[tsenart]

This commit makes zoekt-sourcegraph-indexserver ask for a redirect to
from the internal frontend git tar endpoint to the new gitserver archive
endpoint.

Part of https://github.com/sourcegraph/sourcegraph/issues/4949

[keegancsmith]

I commented in the other PR, but why do we opt-in to this behaviour? Can't we just always do the redirect?

[tsenart]

I commented in the other PR, but why do we opt-in to this behaviour? Can't we just always do the redirect?

We can, but I wanted to make this change safe to roll-out. We should deploy the frontend and git-server first, then zoekt.

In customer deployments, it's possible that zoekt and the frontend get deployed first, and git-server last. In that scenario redirects to /archive wouldn't work, but that'd be temporary and recover quickly.

[keegancsmith]

You should probably just control that in sourcegraph-frontend rather than the client opting in and exposing this implementation detail here. TBH given how we do releases to customers there isn't a nice way to do this without overengineering. So I would just always rely on the redirect, or have a temporary env var for sourcegraph-frontend to pick the codepath, and that way we can experiment.

[tsenart]

You should probably just control that in sourcegraph-frontend rather than the client opting in and exposing this implementation detail here.

Yeah, I realise that there's no difference. We don't need this change. If the frontend we hit has already been upgraded, then we'll follow the redirect in the response. Otherwise, we use the old behaviour.