Fix some const-ness errors noted by C. Mike Pilato <cmpilato@collab.net>
Fix some const-ness errors noted by C. Mike Pilato <cmpilato@collab.net>

* libsvn_fs/dag.c

  (dir_entry_id_from_node): constify the 'id_p' return value
  (svn_fs__dag_link): constify the 'entry_id' variable
  (svn_fs__dag_open): constify the 'node_id' variable

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