Use merge request HEAD ref for detached merge request pipelines

Closes #58454

See merge request gitlab-org/gitlab-ce!25504

By using `refs/merge-requests/:iid/head`

ok

ok

Improve naming nicely

Add nice tests

add nice tests

fix some more

revert

For a given merge request, it's quite common to see duplicate FindCommit
Gitaly requests because the Gitaly CommitService caches the request by
the commit SHA, not by the ref name. However, most of the duplicate
requests use the ref name, so the cache is never actually used in
practice. This leads to unnecessary requests that slow performance.

This commit allows certain callers to bypass the ref name to
OID conversion in the cache. We don't do this by default because it's
possible the tip of the branch changes during the commit, which
would cause the caller to get stale data.

This commit also forces the Ci::Pipeline to use the full ref name
so that caching can work for merge requests.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/57083

This commit updates the pipeline block that appears on the merge request
page to include information that will be exposed by the API as part of
the post-merge pipeline feature.

The flag is on by default, but allows us to revert back
to the old behaviour if we encounter any problems.

Introduces the concept of Prerequisites for a CI build.
If a build has unmet prerequisites it will go through the
:preparing state before being made available to a runner.

There are no actual prerequisites yet, so current
behaviour is unchanged.

Introduces a new status for builds between :created and :pending
that will be used when builds require one or more prerequisite
actions to be completed before being picked up by a runner
(such as creating Kubernetes resources before deploying).

The existing :created > :pending transition is unchanged, so
only builds that require preparation will use the :preparing
status.

Add spec

Fix

Fix

Add changelog

Drop attached

Remove attached

Update changelog

ok

Make it sane

Include merge ref head

Fix union

Improve a bit

Add spec

remove

add spec

Add changelog

fix coding offence

Apply suggestion to spec/models/merge_request_spec.rb

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ok

Fix

Fix spec

Fix spec

fix

Simplify the things

Memoize

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