Adds a conditional to not uncheck an unchecked MR

Changelog: fixed

app/services/merge_requests/update_service.rb

@@ -49,7 +49,7 @@ module MergeRequests
       #
       if merge_request.previous_changes.include?('target_branch') ||
           merge_request.previous_changes.include?('source_branch')
-        merge_request.mark_as_unchecked
+        merge_request.mark_as_unchecked unless merge_request.unchecked?
       end
     end
 

spec/services/merge_requests/update_service_spec.rb

@@ -768,6 +768,13 @@ RSpec.describe MergeRequests::UpdateService, :mailer do
 
           update_merge_request({ target_branch: 'target' })
         end
+
+        it "does not try to mark as unchecked if it's already unchecked" do
+          expect(merge_request).to receive(:unchecked?).and_return(true)
+          expect(merge_request).not_to receive(:mark_as_unchecked)
+
+          update_merge_request({ target_branch: "target" })
+        end
       end
 
       context 'when auto merge is enabled and target branch changed' do