Fix auto-MR-close text from branch name

Rails's form helpers use the `$attr_before_type_cast` method where
available, and this value only appears to be updated on assignment, not
when the object is mutated in some other way:

    [1] pry(main)> mr = MergeRequest.new
    => #<MergeRequest:0x007fcf28395d88 ...>
    [2] pry(main)> mr.description = 'foo'
    => "foo"
    [3] pry(main)> mr.description << ' bar'
    => "foo bar"
    [4] pry(main)> mr.description
    => "foo bar"
    [5] pry(main)> mr.description_before_type_cast
    => "foo"
    [6] pry(main)> mr.description += ' bar'
    => "foo bar bar"
    [7] pry(main)> mr.description_before_type_cast
    => "foo bar bar"

CHANGELOG

@@ -13,6 +13,7 @@ v 8.9.0 (unreleased)
   - Fix endless redirections when accessing user OAuth applications when they are disabled
   - Allow enabling wiki page events from Webhook management UI
   - Bump rouge to 1.11.0
+  - Fix MR-auto-close text added to description
   - Fix issue with arrow keys not working in search autocomplete dropdown
   - Fix an issue where note polling stopped working if a window was in the
     background during a refresh.

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