Merge branch 'fix-auto-mr-close' into 'master'

Fix auto-MR-close text from branch name

See merge request !4836
Signed-off-by: Rémy Coutable <remy@rymai.me>

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 
 v 8.9.1 (unreleased)
+  - Fix MR-auto-close text added to description. !4836
 
 v 8.9.0
   - Fix builds API response not including commit data

app/services/merge_requests/build_service.rb

@@ -83,7 +83,7 @@ module MergeRequests
         closes_issue = "Closes ##{iid}"
 
         if merge_request.description.present?
-          merge_request.description << closes_issue.prepend("\n")
+          merge_request.description += closes_issue.prepend("\n")
         else
           merge_request.description = closes_issue
         end