On fsfs-lock-many branch, fix a bug that was causing hashes to be
removed from the "/" digest one-by-one rather than as a set.

* subversion/libsvn_fs_fs/lock.c
  (unlock_body): Special handling for "/".


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsfs-lock-many@1572000 13f79535-47bb-0310-9956-ffa450edef68