Clear cache to prevent listing deleted branches after MR removes source branch

Closes #2272

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 v 7.14.0 (unreleased)
+  - Clear cache to prevent listing deleted branches after MR removes source branch (Stan Hu)
   - Provide more feedback what went wrong if HipChat service failed test (Stan Hu)
   - Disable turbolinks when linking to Bitbucket import status (Stan Hu)
   - Fix broken code import and display error messages if something went wrong with creating project (Stan Hu)

app/models/repository.rb

@@ -148,6 +148,10 @@ class Repository
     @lookup_cache ||= {}
   end
+  def expire_branch_names
+    cache.expire(:branch_names)
+  end
+
   def method_missing(m, *args, &block)
     if m == :lookup && !block_given?
       lookup_cache[m] ||= {}

lib/gitlab/satellite/merge_action.rb

@@ -36,6 +36,7 @@ module Gitlab
             if merge_request.remove_source_branch?
               # will raise CommandFailed when push fails
               merge_repo.git.push(default_options, :origin, ":#{merge_request.source_branch}")
+              merge_request.source_project.repository.expire_branch_names
             end
             # merge, push and branch removal successful
             true

spec/lib/gitlab/satellite/merge_action_spec.rb

@@ -101,4 +101,18 @@ describe 'Gitlab::Satellite::MergeAction' do
       end
     end
   end
+
+  describe '#merge!' do
+    let(:merge_request) { create(:merge_request, source_project: project, target_project: project, source_branch: "markdown", should_remove_source_branch: true) }
+    let(:merge_action) { Gitlab::Satellite::MergeAction.new(merge_request.author, merge_request) }
+
+    it 'clears cache of source repo after removing source branch' do
+      project.repository.expire_branch_names
+      expect(project.repository.branch_names).to include('markdown')
+
+      merge_action.merge!
+
+      expect(project.repository.branch_names).not_to include('markdown')
+    end
+  end
 end