Simplify / unify the FSFS changed paths list folding logic.

* subversion/libsvn_fs_fs/transaction.c
  (replace_change): Remove this change.
  (path_change_dup): New duplication utility.
  (fold_change): Use the new copy function to create the new change
                 list entries.

Suggested by: julianfoad

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1631115 13f79535-47bb-0310-9956-ffa450edef68