Revert r13208, whose log message was the ever-so-insightful:
Revert r13208, whose log message was the ever-so-insightful:

   * subversion/libsvn_fs_base/tree.c
     (merge, svn_fs_base__commit_txn): Use an iteration subpool.

According to Philip Martin, enabling pool debugging shows that with
this change in place, fs-test 23 (merging_commit) SEGFAULTs, and
without this change it doesn't.

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion