Memoize GitlabShellAdapter for performance and ease of testing

Port of https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/3463#note_47990536

lib/gitlab/shell_adapter.rb

@@ -5,7 +5,7 @@
 module Gitlab
   module ShellAdapter
     def gitlab_shell
-      Gitlab::Shell.new
+      @gitlab_shell ||= Gitlab::Shell.new
     end
   end
 end