Speed up `all_commit_shas` for new merge requests

For merge requests created after 9.4, we have a `merge_request_diff_commits`
table we can get all the SHAs from very quickly. We just need to exclude these
when we load from the legacy format, by ignoring diffs with no serialised
commits.

Once these have been migrated in the background, every MR will see this
improvement.

app/models/merge_request.rb

@@ -849,7 +849,10 @@ class MergeRequest < ActiveRecord::Base
   #
   def all_commit_shas
     if persisted?
-      merge_request_diffs.preload(:merge_request_diff_commits).flat_map(&:commit_shas).uniq
+      column_shas = MergeRequestDiffCommit.where(merge_request_diff: merge_request_diffs).pluck('DISTINCT(sha)')
+      serialised_shas = merge_request_diffs.where.not(st_commits: nil).flat_map(&:commit_shas)
+
+      (column_shas + serialised_shas).uniq
     elsif compare_commits
       compare_commits.to_a.reverse.map(&:id)
     else

changelogs/unreleased/speed-up-merge-request-all-commits-shas.yml

+---
+title: Make loading new merge requests (those created after the 9.4 upgrade) faster
+merge_request:
+author:

spec/models/merge_request_spec.rb

@@ -803,7 +803,7 @@ describe MergeRequest, models: true do
       shared_examples 'returning all SHA' do
         it 'returns all SHA from all merge_request_diffs' do
           expect(subject.merge_request_diffs.size).to eq(2)
-          expect(subject.all_commit_shas).to eq(all_commit_shas)
+          expect(subject.all_commit_shas).to match_array(all_commit_shas)
         end
       end