tasks: Migrate load-git task message format
Pros:
- This aligns the current behavior with other listers and loaders.
- Reading task information will become clearer.
- One step closer to worker's task unification. And possibly refactor a bit the scheduler's model (add a new column field url or something to ease introspection and admin queries)
Cons:
- scheduler's recurring load-git tasks will need to be migrated (or some other way of maintenance if reasonable)
Test Plan
tox
Migrated from D2409 (view on Phabricator)