Fix an incredibly unlikely bug when creating transaction directories in fsfs.
Fix an incredibly unlikely bug when creating transaction directories in fsfs.

* subversion/libsvn_fs_fs/fs_fs.c
  (create_txn_dir): Error out if we run out of unique names.

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion