Significant I/O and CPU load reduction for 'svn log -g'. Repositories
with sub-tree merge info seem to benefit from this the most.

* subversion/libsvn_repos/log.c
  (get_combined_mergeinfo_changes): exit early when there is no
   svn:mergeinfo change in REV

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1481596 13f79535-47bb-0310-9956-ffa450edef68