This `preparing` state will be used later on to determine whether
a merge request is mergeable or not
(via `MergeRequest#mergeable?`).

We also won't check mergeability when the `merge_status` is set
to `preparing`.

We set the `merge_status` to `unchecked` in
`MergeRequests::AfterCreateService` (which is called in
`NewMergeRequestWorker`) so we can check for mergeability once
everything's prepared.

Right now, no merge requests will have their `merge_status` to
`preparing` to keep backwards compatibility and to not result to
incidents/downtimes on multi-stage deploy (e.g. canary).

The `MergeRequest#mark_as_preparing` will be later called in
`MergeRequests::CreateService`. This will trigger the behavior
that we want to achieve with the `preparing` state.