Tweak some merge code. No functional change.

* subversion/libsvn_client/merge.c
  (subtree_touched_by_merge): Tweak the doc string to fix wrong references.
    Note an obscurity. Change a parameter to give access to less data.
  (flag_subtrees_needing_mergeinfo,
   record_mergeinfo_for_dir_merge): Adjust callers.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1437725 13f79535-47bb-0310-9956-ffa450edef68