Reduce server-side memory consumption during 'svn log'.

* subversion/libsvn_repos/log.c
  (detect_changed): Since the incomming change hash already lives in
                    POOL, we don't need to duplicate any of that data.
                    New info needs to copied to POOL, though.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1586923 13f79535-47bb-0310-9956-ffa450edef68