Fix bug where manually merged branches in a MR would end up with an empty diff

Closes #3314

CHANGELOG

@@ -1,6 +1,7 @@
 Please view this file on the master branch, on stable branches it's out of date.
 
 v 8.2.0 (unreleased)
+  - Fix bug where manually merged branches in a MR would end up with an empty diff (Stan Hu)
   - Force update refs/merge-requests/X/head upon a push to the source branch of a merge request (Stan Hu)
   - Improved performance of finding users by one of their Email addresses
   - Improved performance of replacing references in comments

app/services/merge_requests/refresh_service.rb

@@ -7,17 +7,17 @@ module MergeRequests
       @branch_name = Gitlab::Git.ref_name(ref)
 
       find_new_commits
+      # Be sure to close outstanding MRs before reloading them to avoid generating an
+      # empty diff during a manual merge
+      close_merge_requests
       reload_merge_requests
 
       # Leave a system note if a branch was deleted/added
       if branch_added? || branch_removed?
         comment_mr_branch_presence_changed
-        comment_mr_with_commits
-      else
-        comment_mr_with_commits
-        close_merge_requests
       end
 
+      comment_mr_with_commits
       execute_mr_web_hooks
 
       true