Geo: Part 3 of Enforce not null merge_request_diffs.external_diff_store

What does this MR do?

Part 1: !38549 (merged)

Part 2: !38719 (merged)

This MR is the last part needed to enforce not null external_diff_store on merge_request_diffs. This improves the performance of queries for locally stored MR diffs since we no longer have to look for 1 and also NULL values.

Part of #235516 (closed)

Migration output

Up:

== 20200910170908 EnsureFilledExternalDiffStoreOnMergeRequestDiffs: migrating =
== 20200910170908 EnsureFilledExternalDiffStoreOnMergeRequestDiffs: migrated (0.0127s) 

== 20200910175553 ValidateNotNullExternalDiffStoreOnMergeRequestDiffs: migrating 
-- execute("ALTER TABLE merge_request_diffs VALIDATE CONSTRAINT check_93ee616ac9;")
   -> 0.0002s
-- transaction_open?()
   -> 0.0000s
-- indexes(:merge_request_diffs)
   -> 0.0029s
-- remove_index(:merge_request_diffs, {:algorithm=>:concurrently, :name=>"index_merge_request_diffs_external_diff_store_is_null"})
   -> 0.0053s
== 20200910175553 ValidateNotNullExternalDiffStoreOnMergeRequestDiffs: migrated (0.0097s)

Down:

== 20200910175553 ValidateNotNullExternalDiffStoreOnMergeRequestDiffs: reverting 
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:merge_request_diffs, :id, {:where=>"external_diff_store IS NULL", :name=>"index_merge_request_diffs_external_diff_store_is_null", :algorithm=>:concurrently})
   -> 0.0037s
-- add_index(:merge_request_diffs, :id, {:where=>"external_diff_store IS NULL", :name=>"index_merge_request_diffs_external_diff_store_is_null", :algorithm=>:concurrently})
   -> 0.0121s
== 20200910175553 ValidateNotNullExternalDiffStoreOnMergeRequestDiffs: reverted (0.0162s) 

== 20200910170908 EnsureFilledExternalDiffStoreOnMergeRequestDiffs: reverting =
== 20200910170908 EnsureFilledExternalDiffStoreOnMergeRequestDiffs: 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.