This cleanup is needed after we fixed a bug related to the deletion of
draft comments refs (e.g. after being published or deleted). More
context in I6f7b65828b.
This bug caused some draft comments refs to remain in the git "All-Users"
repo. These refs point to an empty tree which has a unique SHA1 in git.
This also kept the history of commits pointed to by these refs through
the parent pointers because updates to these refs were made using
fast-forward transactions.
The empty tree SHA1 was used to detect and delete these zombie refs.
Change-Id: I6aaf4ac292aca2da52325dffd9dbce74c0510390
c14217757f