MergeRequests::PushOptionsHandlerService has been updated to allow
creating and updating merge requests with the
`merge_when_pipeline_succeeds` set using git push options.

To create a new merge request and set it to merge when the pipeline
succeeds:

  git push -u origin -o merge_request.create \
    -o merge_request.merge_when_pipeline_succeeds

To update an existing merge request and set it to merge when the
pipeline succeeds:

  git push -u origin -o merge_request.merge_when_pipeline_succeeds

Issue https://gitlab.com/gitlab-org/gitlab-ce/issues/53198

Previously the raw push option Array was sent to Pipeline::Chain::Skip.

This commit updates this class (and the chain of classes that pass the
push option parameters from the API internal `post_receive` endpoint to
that class) to treat push options as a Hash of options parsed by
GitLab::PushOptions.

The GitLab::PushOptions class takes options like this:

    -o ci.skip -o merge_request.create -o merge_request.target=branch

and turns them into a Hash like this:

    {
      ci: {
        skip: true
      },
      merge_request: {
        create: true,
        target: 'branch'
      }
    }

This now how Pipeline::Chain::Skip is determining if the `ci.skip` push
option was used.

To create a new merge request:

  git push -u origin -o merge_request.create

To create a new merge request setting target branch:

  git push -u origin -o merge_request.create \
    -o merge_request.target=123

To update an existing merge request with a new target branch:

  git push -u origin -o merge_request.target=123

A new Gitlab::PushOptions class handles parsing and validating the push
options array. This can be the start of the standard of GitLab accepting
push options that follow namespacing rules. Rules are discussed in issue
https://gitlab.com/gitlab-org/gitlab-ce/issues/43263.

E.g. these push options:

  -o merge_request.create -o merge_request.target=123

Become parsed as:

  {
    merge_request: {
      create: true,
      target: '123',
    }
  }

And are fetched with the class via:

  push_options.get(:merge_request)
  push_options.get(:merge_request, :create)
  push_options.get(:merge_request, :target)

A new MergeRequests::PushOptionsHandlerService takes the `merge_request`
namespaced push options and handles creating and updating
merge requests.

Any errors encountered are passed to the existing `output` Hash in
Api::Internal's `post_receive` endpoint, and passed to gitlab-shell
where they're output to the user.

Issue https://gitlab.com/gitlab-org/gitlab-ce/issues/43263

Backport `add_default_project_creation_setting` to CE

See merge request gitlab-org/gitlab-ce!27141

Fix stage index migration failing in PostgreSQL 10

Closes #59985

See merge request gitlab-org/gitlab-ce!26972

Refactor Issues doc

Closes #56877

See merge request gitlab-org/gitlab-ce!24695

Document IS_GITLAB_EE environment variable

Closes #60131

See merge request gitlab-org/gitlab-ce!27120

Enable ESLint rule jest/no-focused-tests

See merge request gitlab-org/gitlab-ce!27132

Override problematic display with utility class

Closes #60231

See merge request gitlab-org/gitlab-ce!27138

GraphQL complexity limit too low for Schema load / IntrospectionQuery

Closes #60123

See merge request gitlab-org/gitlab-ce!27063

CE: Extract EE specific files/lines for Discussion spec/services

See merge request gitlab-org/gitlab-ce!26950

Edits to documentation dev doc index

See merge request gitlab-org/gitlab-ce!27135

An IntrospectionQuery required more
complexity points.

Fix typo “settings” in Merge requests docs

See merge request gitlab-org/gitlab-ce!27020

Improvements to Documentation style and structure information

See merge request gitlab-org/gitlab-ce!27098

Add test suite with PostgreSQL 10 to be run on a schedule.

Also, move MySQL tests to run only on master branch, and on schedules.

See merge request gitlab-org/gitlab-ce!26920

Also, move MySQL tests to run only on master branch, and on schedules.
Signed-off-by: Balasankar "Balu" C <balasankar@gitlab.com>

Improve performance of PR import

See merge request gitlab-org/gitlab-ce!27121

Extract a Git::HooksService

Closes #15463

See merge request gitlab-org/gitlab-ce!26726

Fix `updated_at` doesn't apply to `state_event` updates of issues via API.

Closes #51279 and #46980

See merge request gitlab-org/gitlab-ce!27124

Externalize strings in flash messages

See merge request gitlab-org/gitlab-ce!27106

- Externalize strings in controllers
- Update PO file

This removes unneeded `.reload` call which
makes AR to load ALL objects, and create its
in-memory representation.

Add useful tips about big repositories

See merge request gitlab-org/gitlab-ce!27005

Adds EE folder in the stylelint command

See merge request gitlab-org/gitlab-ce!27058

Display maximum artifact size from runtime config

See merge request gitlab-org/gitlab-ce!26784

Docs: Fix anchors related to variables doc

See merge request gitlab-org/gitlab-ce!27081

Add doc when rspec-set won't work

See merge request gitlab-org/gitlab-ce!27050

Update dependency @gitlab/svgs to ^1.59.0

See merge request gitlab-org/gitlab-ce!27104