Reduce the memory consumption of "svn log -v" by ~20%.

* subversion/libsvn_repos/log.c
  (detect_changed): Don't construct a copy of the CHANGES hash if possible.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1605195 13f79535-47bb-0310-9956-ffa450edef68