Use the new mergeinfo-mod flag to speed up other loggy operations.

* subversion/libsvn_repos/rev_hunt.c
  (get_merged_mergeinfo): Shortcut if we know for sure that there is no
                          mergeinfo change.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1560112 13f79535-47bb-0310-9956-ffa450edef68