As part of the effort towards creating a registry for SCMTools, we're

working towards deprecating the old Tool model, which was what we had

initially created to support multiple SCM types. This model was

initially populated via a database fixture, and over the years we've

moved to entry points, and (soon) to a registry for storing all of the

available tools.
If Tool is going away over time, we need a new way for Repository to

keep track of what SCMTool it's supposed to use. This change adds a new

field called scmtool_id, which will be the ID that can be looked up in

the registry. Populating this field is a little complex because the

existing Tool object only stores the SCMTool class name, not the ID. We

therefore can't initialize the field from an evolution, even using

custom SQL. The solution we've settled on is to wrap the upgrade process

with something that will store a mapping of the existing ID relations,

run any database evolutions, and then apply the mapping to populate the

field.
This should allow us to move that information from the Tool model (and

associated SCMTool class) into the Repository model during the upgrade,

even in the case where the evolutions being run delete the Tool model

entirely. For now the code that builds that stored info uses the Tool

model, but there's a note about how we'll need to update that to use

hand-written SQL queries when the model instance gets deleted in the

future.

Set up a bunch of different repositories and ran manage.py upgrade.

Saw that the new field was populated correctly.