Add comments.

* subversion/libsvn_repos/rev_hunt.c
  (path_revision,
   get_merged_mergeinfo): Explain what the mergeinfo 'difference' really is.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1628685 13f79535-47bb-0310-9956-ffa450edef68