Fix upgrader

It created branch based on current branch which is an old version
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

lib/gitlab/upgrader.rb

@@ -51,7 +51,7 @@ module Gitlab
       {
         "Stash changed files" => "git stash",
         "Get latest code" => "git fetch",
-        "Switch to new version" => "git checkout -b v#{latest_version}",
+        "Switch to new version" => "git checkout v#{latest_version}",
         "Install gems" => "bundle",
         "Migrate DB" => "bundle exec rake db:migrate RAILS_ENV=production",
         "Recompile assets" => "bundle exec rake assets:clean assets:precompile RAILS_ENV=production",