On the 'issue-3550-dev' branch, fix some brokeness of the changes
duplicating code.  (I introduced said brokeness in r890955.)

* subversion/libsvn_fs_base/revs-txns.c
  (changes_dup): Add 'old_txn_id' and 'new_txn_id' parameters, reorder
    the other parameters some, and de-confuse the use of `changes'
    keys and transaction IDs in the logic.
  (txn_body_begin_obliteration_txn): Update call to changes_dup().


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/issue-3550-dev@892311 13f79535-47bb-0310-9956-ffa450edef68