Merge branch 'fix-issue-3138' into 'master'

Force update refs/merge-requests/X/head upon a push to the source branch of a merge request

If a user rebases and does a force push, GitLab would not update the `refs/merge-requests/X/head` link. Using the -f flag forces this to happen.

Closes #3138

See merge request !1683

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
+v 8.1.3
+  - Force update refs/merge-requests/X/head upon a push to the source branch of a merge request (Stan Hu)
+
 v 8.1.2
   - Fix cloning Wiki repositories via HTTP (Stan Hu)
   - Add migration to remove satellites directory

app/models/repository.rb

@@ -512,7 +512,7 @@ class Repository
   end
   def fetch_ref(source_path, source_ref, target_ref)
-    args = %W(git fetch #{source_path} #{source_ref}:#{target_ref})
+    args = %W(git fetch -f #{source_path} #{source_ref}:#{target_ref})
     Gitlab::Popen.popen(args, path_to_repo)
   end