In wc_db's relocate code: do everything inside a single transaction.

* subversion/libsvn_wc/wc_db.c
  (relocate_txn): Integrate some code that used to be...
  (svn_wc__db_global_relocate): ... in its single caller. Add missing
    entries flush.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1545129 13f79535-47bb-0310-9956-ffa450edef68