Fix default branch being cached and show old one after change

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/contexts/projects/update_context.rb

@@ -5,12 +5,14 @@ module Projects
       params[:project].delete(:public) unless can?(current_user, :change_public_mode, project)
       new_branch = params[:project].delete(:default_branch)
 
-      if project.repository.exists? && new_branch != project.repository.root_ref
+      if project.repository.exists? && new_branch != project.default_branch
         GitlabShellWorker.perform_async(
           :update_repository_head,
           project.path_with_namespace,
           new_branch
         )
+
+        project.reload_default_branch
       end
 
       project.update_attributes(params[:project], as: role)

app/models/project.rb

@@ -451,4 +451,9 @@ class Project < ActiveRecord::Base
   def default_branch
     @default_branch ||= repository.root_ref if repository.exists?
   end
+
+  def reload_default_branch
+    @default_branch = nil
+    default_branch
+  end
 end