Follow-up to r1648253:  Fix pool usage issue.

* subversion/libsvn_fs_fs/tree.c
  (find_youngest_copyroot): Document that the result will not be allocated
                            in POOL.
  (fs_closest_copy): Call the above with our subpool as scratch pool and
                     copy the final path into our result pool.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1648272 13f79535-47bb-0310-9956-ffa450edef68