[Bug]: What are branch Prisma databases for if the preview database is used for branch deployments?

### Problem description

While using Deno Deploy with Prisma, I noticed that the Pre-deploy command uses a the branch-specific database URL while the preview deployment uses the preview database.

This is someone documented:

> Currently only one preview database is created per app, shared across all preview deployments. In future releases, each preview deployment will get its own isolated database.

Is this configurable though? Is there a way to chose which database preview deployments from a specific branch use?

### Steps to reproduce

N/A

### Expected behavior

Branch previews would use the branch specific database.

No files or tests are named. Start by reviewing how Deno Deploy maps the pre-deploy command and preview deployments to database URLs, then determine whether branch selection is configurable; done means documenting the supported behavior or specifying the required configuration.

Written by the indexing model from the issue text.