Prevent unnecessary memory usage peaks when reading changed path lists
in tight memory situations.

* subversion/libsvn_fs_fs/cached_data.c
  (svn_fs_fs__get_changes): Serialize the list only if it fits into cache
                            at all.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1605191 13f79535-47bb-0310-9956-ffa450edef68