Set merge_request_diff_id on MR when creating

Once we migrate existing MRs to have this column, we will be able to get the
latest diff for a single merge request more efficiently, and (more importantly)
get all latest diffs for a collection of MRs efficiently.

app/models/merge_request_diff.rb

@@ -48,6 +48,10 @@ class MergeRequestDiff < ActiveRecord::Base
   # Collect information about commits and diff from repository
   # and save it to the database as serialized data
   def save_git_content
+    MergeRequest
+      .where('id = ? AND COALESCE(latest_merge_request_diff_id, 0) < ?', self.merge_request_id, self.id)
+      .update_all(latest_merge_request_diff_id: self.id)
+
     ensure_commit_shas
     save_commits
     save_diffs

changelogs/unreleased/37631-add-a-merge_request_diff_id-column-to-merge_requests.yml

 ---
-title: Make finding most recent merge request diffs more efficient
+title: Add a latest_merge_request_diff_id column to merge_requests
 merge_request: 15035
 author:
 type: performance

lib/gitlab/import_export/import_export.yml

@@ -113,7 +113,7 @@ excluded_attributes:
     - :milestone_id
     - :ref_fetched
     - :merge_jid
-    - :merge_request_diff_id
+    - :latest_merge_request_diff_id
   award_emoji:
     - :awardable_id
   statuses: