Allow branching to and from different instances

Right now, the `branch create` command only operates on the connected instance. It would be useful to be able to specify different instances for the source and target so you can, for instance, copy just the schema of your production database into CI without having to manually create a schema-only branch and use dump->restore, or having to run all of the migrations.

Start at the `branch create` command and trace how it selects the connected instance. Define how source and target instances should be specified, then verify that a schema-only branch can be copied from one instance to another without manual dump-and-restore or rerunning migrations.

Written by the indexing model from the issue text.