Missing timeout on git archive

Created by: chrismwendt

https://github.com/sourcegraph/sourcegraph/pull/30884 inadvertently removed the default timeout on the git archive call here:

https://github.com/sourcegraph/sourcegraph/blob/aa368f7a054d44973f1cae7cd17056f82391daad/cmd/gitserver/server/server.go#L883-L897

This kind of mistake is perhaps what @keegancsmith had in mind when he suggested NoTimeout as the default.