Re-Connecting erases environment variables from App

Steps to reproduce:
1. Create App from a Github repo/branch
2. Set some environment variables
3. In the settings/general disconnect the branch so there are no branches
4. Go to the App's main screen (with Frontend | Backend tabs etc) and see the offer to Connect a frontend web app
5. Follow the Github flow

The environment variables will be lost. I think this is because this flow is really the "new app" flow with some steps elided if the app already exists. But the resulting call to updateApp wipes the env vars.

Note that if you just connect a branch again from settings/general page, it's fine - that flow doesn't try to update the App.

Reproduce the flow by disconnecting the branch, then reconnecting it from the app's main screen after setting environment variables. Trace the resulting updateApp call and compare it with the settings/general connection flow; done means reconnecting preserves the existing environment variables.

Written by the indexing model from the issue text.