Geo: Part 2 of Enforce not null merge_request_diffs.external_diff_store

What does this MR do?

Part of #229412 (closed):

When sync_object_storage is false, RegistryConsistencyService calls methods which use the clause WHERE file_store = 1. We need to:

• index that field (which we've done).
• make it non-nullable.

  • So we don't have to add OR file_store IS NULL to ensure we process all records. That OR harms performance (see the performance improvement when we removed this clause from LFS objects).

• Update existing NULLs to local store:
  • Post-deployment migration scheduling background jobs to update batches of merge_request_diffs with NULL external_diff_store to the LOCAL value

For more context, part 1 is here !38549 (merged).

Migration output

Up:

== 20200805152108 MigrateNullExternalDiffStoreToLocalValue: migrating =========
== 20200805152108 MigrateNullExternalDiffStoreToLocalValue: migrated (0.0062s) 

Down:

== 20200805152108 MigrateNullExternalDiffStoreToLocalValue: reverting =========
== 20200805152108 MigrateNullExternalDiffStoreToLocalValue: reverted (0.0000s) 

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.