fix: MR /merge quick action

This MR sends the mergeRequestDiffHeadSha parameter when we submit a new MR comment. Thanks to this parameter, the /merge quick action will work. The parameter was introduced in this epic &7648.

This parameter is only supported in GitLab 14.9 and newer.

Unfortunately, even a successful merge produces an error thanks to gitlab#346557. So this is only a partial fix.

So the API performs the merge, but the API responds with

{
  "data": {
    "createNote": {
      "errors": [
        "Commands only Merged this merge request."
      ],
      "note": null
    }
  }
}

Related to #545 (closed)