Use simple path for tmp ref, avoiding extra dir

So that we don't have to worry about cleaning empty
directories later. This was brought up at:

https://gitlab.com/gitlab-org/gitlab-ce/issues/38689#note_42242988

lib/gitlab/git/repository.rb

@@ -990,7 +990,7 @@ module Gitlab
         tmp_ref = fetch_ref(
           start_repository,
           source_ref: "#{Gitlab::Git::BRANCH_REF_PREFIX}#{start_branch_name}",
-          target_ref: "refs/tmp/#{SecureRandom.hex}/head"
+          target_ref: "refs/tmp/#{SecureRandom.hex}"
         )
 
         yield commit(sha)