This allows us to take advantage of
https://gitlab.com/gitlab-org/gitlab_git/merge_requests/121.

If `git gc` runs and `Repository` has an instance to `Rugged::Repository`, a
bug in libgit2 may cause the instance to return a stale value or a missing
branch. This change not only optimizes the branch lookup so we don't have
to iterate through every branch, but it also works around the `git gc`
issue by forcing a repository reload every time `Repository#find_branch`
is called.

See: https://github.com/libgit2/libgit2/issues/1534

Closes #15392, #21470

Update doorkeeper to 4.2.0

Changelog: https://git.io/v6PnV

See merge request !5881

Changelog: https://git.io/v6PnV

Update Hamlit to 2.6.1

Fixes gitlab-org/gitlab-ce#21025 and gitlab-org/gitlab-ce#21017

See merge request !5873

Support slash commands in issues / MR description & comments

See merge request !5021

Fix Ace syntax highlighting with compiled assets

## What does this MR do?

Update ACE to 4.1.0 to allow modes (syntax highlighting files for particular languages) to be lazily loaded.

## Are there points in the code the reviewer needs to double check?

Don't think so.

## Why was this MR needed?

Syntax highlighting in the file editor only worked in development and test modes, not in production!

## What are the relevant issue numbers?

Closes #3225.

## Screenshots (if relevant)

First, the test setup - add these lines to `development.rb`:
```ruby
  config.assets.debug = false
  config.assets.compile = false
  config.assets.digest = true
```

Then, before starting the server, run `bundle exec rake assets:clobber assets:precompile`.

Before:

![image](/uploads/486198e273019f8969d2e90560d82928/image.png)

There is an error in the console because `/assets/ace/mode-ruby.js` was not found (it's only available when assets are compiled on demand).

After:

![image](/uploads/6aa0d69b7efdfd6fe6aa22b9a49e9716/image.png)

This loads `/assets/ace/mode-ruby-3915f95a6cc47306b1305e4dcb7aca25b2ef9c49b18ec6011707135b6575d8f3.js`, which works because the precompile step included it and told ACE where to find it.

## 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~~
  - [x] 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 !5501

Fixes gitlab-org/gitlab-ce#21025 and gitlab-org/gitlab-ce#21017