repo-updater: Optimise EnabledStateDeprecationMigration
Created by: tsenart
This commit optimises the EnabledStateDeprecationMigration
so that it
only sources repos from the code host the first time it runs, when there
are still disabled repos stored in the database.
In subsequent runs, sourcing doesn't occur, because the disabled repos have been already deleted and added to the exclude lists of the relevant external services.
This makes repo-updater startup much faster.
Supersedes #3912
Test plan: go test