Hide remove source branch button when new commit is added to branch

Fixes #3339

This MR hides the 'Remove source branch' button when a new commit is added to the source branch

CHANGELOG

@@ -28,6 +28,7 @@ v 8.5.0 (unreleased)
   - Support Akismet spam checking for creation of issues via API (Stan Hu)
   - Improve UI consistency between projects and groups lists
   - Add sort dropdown to dashboard projects page
+  - Hide remove source branch button when the MR is merged but new commits are pushed (Zeger-Jan van de Weg)
 v 8.4.3
   - Increase lfs_objects size column to 8-byte integer to allow files larger

app/models/merge_request.rb

@@ -284,7 +284,8 @@ class MergeRequest < ActiveRecord::Base
   def can_remove_source_branch?(current_user)
     !source_project.protected_branch?(source_branch) &&
       !source_project.root_ref?(source_branch) &&
-      Ability.abilities.allowed?(current_user, :push_code, source_project)
+      Ability.abilities.allowed?(current_user, :push_code, source_project) &&
+      last_commit == source_project.commit(source_branch)
   end
   def mr_and_commit_notes

spec/models/merge_request_spec.rb

@@ -226,9 +226,15 @@ describe MergeRequest, models: true do
       expect(subject.can_remove_source_branch?(user2)).to be_falsey
     end
-    it "is can be removed in all other cases" do
+    it "can be removed if the last commit is the head of the source branch" do
+      allow(subject.source_project).to receive(:commit).and_return(subject.last_commit)
+
       expect(subject.can_remove_source_branch?(user)).to be_truthy
     end
+
+    it "cannot be removed if the last commit is not also the head of the source branch" do
+      expect(subject.can_remove_source_branch?(user)).to be_falsey
+    end
   end
   describe "#reset_merge_when_build_succeeds" do