Github importer Avoid touching MR when importing merged by

When importing a Pull Request `merged by` field, if the user cannot be
mapped to a Gitlab user, a note is created, which `touch` the
MergeRequest (callback). This callback can be skipped by setting the
`importing` attribute of the note to `true`.

Related to: https://gitlab.com/gitlab-org/gitlab/-/issues/289671

changelogs/unreleased/kassio-github-importer-avoid-touch-mr-when-importing-merged-by.yml

+---
+title: Github importer - Avoid touching MR when importing pull request `merged by` field
+merge_request: 48729
+author:
+type: fixed

lib/gitlab/github_import/importer/pull_request_merged_by_importer.rb

@@ -26,6 +26,7 @@ module Gitlab
             }, unique_by: :merge_request_id)
           else
             merge_request.notes.create!(
+              importing: true,
               note: "*Merged by: #{pull_request.merged_by.login}*",
               author_id: project.creator_id,
               project: project,

spec/lib/gitlab/github_import/importer/pull_request_merged_by_importer_spec.rb

@@ -28,7 +28,9 @@ RSpec.describe Gitlab::GithubImport::Importer::PullRequestMergedByImporter, :cle
   end
 
   it 'adds a note referencing the merger user when the user cannot be mapped' do
-    expect { subject.execute }.to change(Note, :count).by(1)
+    expect { subject.execute }
+      .to change(Note, :count).by(1)
+      .and not_change(merge_request, :updated_at)
 
     last_note = merge_request.notes.last