Tweak comments.

* subversion/libsvn_fs_fs/transaction.c,
  subversion/libsvn_fs_x/transaction.c
  (replace_change): Remove a comment, wrongly duplicated in r1525419.
  (fold_change): Revert a comment, missed in reverting r1525419 in r1597989.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1628676 13f79535-47bb-0310-9956-ffa450edef68