Note a problem that svn.apache.org runs into occasionally.

* subversion/libsvn_fs_fs/fs_fs.c
  (svn_fs_fs__commit): Note a potential problem with the approach and suggest
    a general solution.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1481577 13f79535-47bb-0310-9956-ffa450edef68