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

Closes #3138

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 
 v 8.2.0 (unreleased)
+  - Force update refs/merge-requests/X/head upon a push to the source branch of a merge request (Stan Hu)
   - Improved performance of replacing references in comments
   - Show last project commit to default branch on project home page
   - Highlight comment based on anchor in URL

app/models/repository.rb

@@ -528,7 +528,7 @@ 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