Significantly reduce I/O during 'svn log -v' and 'svn log' with authz.
There is no need to read the copy-from info from the repository
if it is already available in the changed path list.

* subversion/libsvn_repos/log.c
  (detect_changed): use the copyfrom info of the change, if available

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1481594 13f79535-47bb-0310-9956-ffa450edef68