Replace a few instances of custom path calculations with svn_dirent_is_child().
Replace a few instances of custom path calculations with
svn_dirent_is_child().

* subversion/libsvn_client/merge.c
  (fix_deleted_subtree_ranges,
   inherit_implicit_mergeinfo_from_parent,
   populate_remaining_ranges,
   update_wc_mergeinfo,
   record_mergeinfo_for_dir_merge):
   Use API svn_dirent_is_child() API rather than various home brew versions.

  (drive_merge_report_editor): As above, but require an absolute path target.
   All callers are presently passing absolute paths, so this is just a
   argument rename.

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion