Following up on r1643810, only update the operational log state for the
root of the recursion. This avoids leaking a tiny bit of memory for every
directory walked.

* subversion/mod_dav_svn/repos.c
  (do_walk): Add walk_root variable and use it to avoid changing per request
    operational log state for every node (while growing memory usage).


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1643816 13f79535-47bb-0310-9956-ffa450edef68