Git Flow: coder/coder/v2/site: query coderd for git repo branches

Relates to https://github.com/coder/internal/issues/406 https://github.com/coder/internal/issues/405 and https://github.com/coder/internal/issues/404

As https://github.com/coder/internal/issues/406 but for Coder's Web UI.