Compared to the merge_request_diff association:

1. It's simpler to query. The query uses a foreign key to the
   merge_request_diffs table, so no ordering is necessary.
2. It's faster for preloading. The merge_request_diff association has to load
   every diff for the MRs in the set, then discard all but the most recent for
   each. This association means that Rails can just query for N diffs from N
   MRs.
3. It's more complicated to update. This is a bidirectional foreign key, so we
   need to update two tables when adding a diff record. This also means we need
   to handle this as a special case when importing a GitLab project.

There is some juggling with this association in the merge request model:

* `MergeRequest#latest_merge_request_diff` is _always_ the latest diff.
* `MergeRequest#merge_request_diff` reuses
  `MergeRequest#latest_merge_request_diff` unless:
    * Arguments are passed. These are typically to force-reload the association.
    * It doesn't exist. That means we might be trying to implicitly create a
      diff. This only seems to happen in specs.
    * The association is already loaded. This is important for the reasons
      explained in the comment, which I'll reiterate here: if we a) load a
      non-latest diff, then b) get its `merge_request`, then c) get that MR's
      `merge_request_diff`, we should get the diff we loaded in c), even though
      that's not the latest diff.

Basically, `MergeRequest#merge_request_diff` is the latest diff in most cases,
but not quite all.

Add Group Milestone sorting

Closes #39720

See merge request gitlab-org/gitlab-ce!15230

Add QUERY_RECORDER_DEBUG environment variable to improve performance debugging

See merge request gitlab-org/gitlab-ce!15499

Add support of Mermaid

Closes #3711

See merge request gitlab-org/gitlab-ce!15107

Add FetchSourceBranch Gitaly call

Closes gitaly#758

See merge request gitlab-org/gitlab-ce!15498

[ci skip]

Update composite pipelines index to include "id"

See merge request gitlab-org/gitlab-ce!15519

default fill color for SVGs

See merge request gitlab-org/gitlab-ce!15476

ignore hashed repos (for now) when using `rake gitlab:cleanup:repos`

Closes #40291

See merge request gitlab-org/gitlab-ce!15520

Set the default gitlab-shell timeout to 3 hours

Closes #39977

See merge request gitlab-org/gitlab-ce!15292

Use Redis cache for branch existence checks

Closes #40349

See merge request gitlab-org/gitlab-ce!15513

Update CONTRIBUTING.md: Link definition of done to criteria

See merge request gitlab-org/gitlab-ce!15500

FileUploader should use Hashed Storage only when project is migrated

See merge request gitlab-org/gitlab-ce!15526

Simplify Feature Proposal template

See merge request gitlab-org/gitlab-ce!15297

Use arrays in Ci::Pipeline#latest_builds_with_artifacts

See merge request gitlab-org/gitlab-ce!15525

Use `make install` for Gitaly setups in non-test environments

See merge request gitlab-org/gitlab-ce!15295

Fix blank states using old css

Closes #40377

See merge request gitlab-org/gitlab-ce!15521

Add wiki css to help page

See merge request gitlab-org/gitlab-ce!15488

This updates the composite index on ci_pipelines (project_id, ref,
status) to also include the "id" column at the end. Adding this column
to the index drastically improves the performance of queries used for
getting the latest pipeline for a particular branch. For example, on
project dashboards we'll run a query like the following:

    SELECT ci_pipelines.*
    FROM ci_pipelines
    WHERE ci_pipelines.project_id = 13083
    AND ci_pipelines.ref = 'master'
    AND ci_pipelines.status = 'success'
    ORDER BY ci_pipelines.id DESC
    LIMIT 1;

    Limit  (cost=0.43..58.88 rows=1 width=224) (actual time=26.956..26.956 rows=1 loops=1)
      Buffers: shared hit=6544 dirtied=16
      ->  Index Scan Backward using ci_pipelines_pkey on ci_pipelines  (cost=0.43..830922.89 rows=14216 width=224) (actual time=26.954..26.954 rows=1 loops=1)
            Filter: ((project_id = 13083) AND ((ref)::text = 'master'::text) AND ((status)::text = 'success'::text))
            Rows Removed by Filter: 6476
            Buffers: shared hit=6544 dirtied=16
    Planning time: 1.484 ms
    Execution time: 27.000 ms

Because of the lack of "id" in the index we end up scanning over the
primary key index, then applying a filter to filter out any remaining
rows. The more pipelines a GitLab instance has the slower this will get.

By adding "id" to the mentioned composite index we can change the above
plan into the following:

    Limit  (cost=0.56..2.01 rows=1 width=224) (actual time=0.034..0.034 rows=1 loops=1)
      Buffers: shared hit=5
      ->  Index Scan Backward using yorick_test on ci_pipelines  (cost=0.56..16326.37 rows=11243 width=224) (actual time=0.033..0.033 rows=1 loops=1)
            Index Cond: ((project_id = 13083) AND ((ref)::text = 'master'::text) AND ((status)::text = 'success'::text))
            Buffers: shared hit=5
    Planning time: 0.695 ms
    Execution time: 0.061 ms

This in turn leads to a best-case improvement of roughly 25
milliseconds, give or take a millisecond or two.

This changes Ci::Pipeline#latest_builds_with_artifacts so it returns an
Array instead of a relation. Whenever we use this data we do so in two
steps:

1. Count the number of rows
2. If this number is greater than 0, iterate over the rows

By returning an Array instead we only execute 1 query of which the total
time/work is less than running either just a COUNT(*) or both queries
(in the worst case).

On GitLab.com this change should save us a few milliseconds per request
to ProjectsController#show.

Optimize read-only middleware so that it does not consume as much CPU

Closes #40185 and gitlab-com/infrastructure#3240

See merge request gitlab-org/gitlab-ce!15504

Skip confirmation user api

Closes #28934

See merge request gitlab-org/gitlab-ce!15175

Custom issue tracker

See merge request gitlab-org/gitlab-ce!15516

Resolve "Notes API for issues no longer returns label additions/removals"

Closes #39461

See merge request gitlab-org/gitlab-ce!15080

Resolve "TagsPage|Tags"

Closes #40367

See merge request gitlab-org/gitlab-ce!15512