Merge branch 'memoize_merge_request_events' into 'master'

Memoize MR merged/closed events retrieval

## What does this MR do?

Memoize a database call that is execute 3 consecutive times 

## Why was this MR needed?

To speed up MR pages

## What are the relevant issue numbers?

#14202 , #19490

## Screenshots (if relevant)

The query is fast most of the times, but we can avoid the 3 call counts to the database

![Screen_Shot_2016-07-08_at_12.49.48](/uploads/ab0f6d843be30754a06e0f05bdf2102d/Screen_Shot_2016-07-08_at_12.49.48.png)

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- ~~[ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)~~
- ~~[ ] API support added~~
- Tests
  - ~~[ ] Added for this feature/bug~~
  - [ ] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5151

CHANGELOG

@@ -45,6 +45,7 @@ v 8.10.0 (unreleased)
   - RailsCache metris now includes fetch_hit/fetch_miss and read_hit/read_miss info.
   - Allow [ci skip] to be in any case and allow [skip ci]. !4785 (simon_w)
   - Set import_url validation to be more strict
+  - Memoize MR merged/closed events retrieval
   - Add basic system information like memory and disk usage to the admin panel
   - Don't garbage collect commits that have related DB records like comments
   - More descriptive message for git hooks and file locks

app/models/merge_request.rb

@@ -318,11 +318,11 @@ class MergeRequest < ActiveRecord::Base
   end
 
   def merge_event
-    self.target_project.events.where(target_id: self.id, target_type: "MergeRequest", action: Event::MERGED).last
+    @merge_event ||= target_project.events.where(target_id: self.id, target_type: "MergeRequest", action: Event::MERGED).last
   end
 
   def closed_event
-    self.target_project.events.where(target_id: self.id, target_type: "MergeRequest", action: Event::CLOSED).last
+    @closed_event ||= target_project.events.where(target_id: self.id, target_type: "MergeRequest", action: Event::CLOSED).last
   end
 
   WIP_REGEX = /\A\s*(\[WIP\]\s*|WIP:\s*|WIP\s+)+\s*/i.freeze