createDiffNote GraphQL mutation should use the CreateDiffNoteInput.position.headSha for /merge quick action

In the epic &7648 we explain how /merge quick action wasn't supported for createNote mutation.

The reason was that the merge logic expects us to submit merge_request_head_diff_sha. This attribute serves optimistic locking. If we try to merge, but we don't have the latest HEAD_SHA, the quick action will fail.

The related createDiffNote now fails with this error:

{
  "createDiffNote": {
    "errors":["Commands only The `/merge` quick action requires the SHA of the head of the branch."],
    "note":null
  }
}

The createDiffNote mutation takes CreateDiffNoteInput. This input contains DiffPositionInput which contains the headSha parameter.

The quick action logic should be able to use this headSha and not require any changes to the createDiffNote mutation interface.